Allow configuring API server LB rule name and health probe in AzureCluster

[cPu1]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This changelist allows configuring the names for the API server load balancer rule and health probe to support cluster adoption scenarios where existing networking resources already exist with different names.

Currently, CAPZ hardcodes the load balancer rule name (LBRuleHTTPS) and health probe name (HTTPSProbe) for the API server load balancer, making it difficult to adopt existing Kubeadm-based clusters using different naming conventions, as users cannot reuse existing load balancer resources without manual intervention. Since the rule name and probe name cannot be changed without destroying and recreating the resource, adopting a Kubeadm cluster into Cluster API can result in some downtime.

This PR adds two new optional fields LoadBalancingRule.Name and HealthProbe.Name to LoadBalancerSpec.

To ensure backward compatibility, the defaulting webhook uses default values that match the previous hardcoded names (LBRuleHTTPS and HTTPSProbe). Since changing the load balancer rule name and health probe name requires resource recreation, a destructive operation, the validating webhook disallows mutations to these fields.

There are no breaking changes as this is a purely additive change that maintains full backward compatibility.

Please note that there's a known bug caused by reusing the same health probe for AdditionalAPIServerLBPorts but this PR keeps the current behavior unchanged.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #5726

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

Allows configuring API server load balancer rule name and health probe name in `AzureCluster`

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign enxebre for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

Hi @cPu1. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.02%. Comparing base (e859273) to head (b8e4561).
⚠️ Report is 96 commits behind head on main.

Files with missing lines	Patch %	Lines
api/v1beta1/azurecluster_validation.go	72.72%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5828      +/-   ##
==========================================
+ Coverage   46.93%   47.02%   +0.09%     
==========================================
  Files         279      279              
  Lines       29687    29721      +34     
==========================================
+ Hits        13934    13977      +43     
+ Misses      14940    14931       -9     
  Partials      813      813              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[cPu1]

@jsturtevant, @marosset, can you help review this PR?

[jackfrancis]

/ok-to-test

[jackfrancis]

/test pull-cluster-api-provider-azure-e2e-workload-upgrade
/test pull-cluster-api-provider-azure-e2e-optional

[jackfrancis]

@nojnhuh @mboersma generally speaking this looks back-compat-friendly, any concerns there?

[k8s-ci-robot]

@cPu1: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-azure-e2e-optional	b8e4561	link	false	/test pull-cluster-api-provider-azure-e2e-optional

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[nojnhuh]

I don't love that this introduces an assumption that a load balancer will only ever have one rule. If we ever add more, then this field becomes ambiguous. I don't know why we would ever want to do that, but I'm equally unsure that one is all we'll ever want.

This also adds the field for the other kinds of load balancers CAPZ creates (worker/controlplane outbound) but it does nothing in those cases. I would slightly prefer the approach we took with AdditionalAPIServerLBPorts where it lives up a level and specifically applies only to the API server LB:

cluster-api-provider-azure/api/v1beta1/types.go

Lines 114 to 117 in cc24323

	// AdditionalAPIServerLBPorts specifies extra inbound ports for the APIServer load balancer.
	// Each port specified (e.g., 9345) creates an inbound rule where the frontend port and the backend port are the same.
	// +optional
	AdditionalAPIServerLBPorts []LoadBalancerPort `json:"additionalAPIServerLBPorts,omitempty"`