Add workflow for generating release notes

[colleenmcginnis]

What is the problem this PR solves?

This will ensure that for each release, the release notes are always generated using the latest version of elastic-agent-changelog-tool.

How does this PR solve the problem?

Adds a GitHub workflow that can be manually triggered to generate release notes and open a PR.

As currently written, the workflow needs to be manually triggered by the control plane release manager (@ebeahan or @pierrehilbert). They need to provide the SHA of the build candidate for the release and the version number. The team could choose to trigger it another way in the future as long as they can pass it those two pieces of information.

It automatically opens a PR adding the generated changelog.yml file, cleaning up fragments, and adding the relevant Markdown release note files. Here's the outcome from an earlier test in elastic-agent: colleenmcginnis/elastic-agent#2.

A couple notes:

• The @colleenmcginnis in the test PR will be replaced with the username of whoever triggered the job.
• Currently the action won't automatically run the preview build because (I think) using the default GitHub action token restricts you from using that action to trigger another action. I'm not sure if there's an alternative we could use in this repo.

How to test this PR locally

N/A

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues PRs

• Add centralized generate-release-notes workflow elastic-agent-changelog-tool#223
• Add workflow for generating release notes elastic-agent#10931

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