Add client for Connections API, some wiring up the client to the provider

[fridgepoet]

This PR:

• adds a client to reach the Connections API to create metrics endpoint scrape jobs, based on ongoing work from the Cloud Provider O11y squad on AWS scrape jobs
• adds some wiring for the client to be initialized in the provider, provided an access token and url are provided
• fixes up (housekeeping) some errors in the code that were added previously (e.g. value receiver instead of pointer receiver Scaffold metrics endpoint scrape job resource #1807)
• adds to the resource's metrics endpoint scrape URL validation (it also must be https)

This contributes to #7670 and #7671

Special notes:

• metrics endpoint scrape URL validation (must be https)

Alright, I found hashicorp/terraform-plugin-framework-validators#125, maybe they'll make a URL validator.

In the meantime, I added one in the latest commit:

Some examples with a terraform resource:

1. resource has url = ""

Step 1/1 error: Error running pre-apply refresh: exit status 1
        
Error: value must be valid URL with HTTPS

  with grafana_connections_metrics_endpoint_scrape_job.test,
  on terraform_plugin_test.tf line 7, in resource "grafana_connections_metrics_endpoint_scrape_job" "test":
   7:   url = ""

A valid URL is required.

Given Value: ""

2. resource does not have https: url="www.google.com"

Step 1/1 error: Error running pre-apply refresh: exit status 1
        
Error: value must be valid URL with HTTPS

  with grafana_connections_metrics_endpoint_scrape_job.test,
  on terraform_plugin_test.tf line 7, in resource "grafana_connections_metrics_endpoint_scrape_job" "test":
   7:   url = "www.google.com"

A URL was provided, protocol must be HTTPS.

Given Value: "www.google.com"

[github-actions]

In order to lower resource usage and have a faster runtime, PRs will not run Cloud tests automatically.
To do so, a Grafana Labs employee must trigger the cloud acceptance tests workflow manually.

[fridgepoet]

@matthewnolf Do you have any idea about this lint error? https://github.com/grafana/terraform-provider-grafana/actions/runs/10942942082/job/30381152446?pr=1809#step:3:67

Edit: changed to pointer receivers as discussed, thanks Matt