[CI] Pull in the latest mutes from base branch for PRs at runtime

[brianseeders]

This adds the ability for PR CI to pull in the latest muted-tests.yml content from the PR's base branch and merge it with the contents in the branch. This means that when a test is muted, it will immediately take affect in PRs, without them needing to merge upstream. This will hopefully cut down on unnecessary CI failures for PRs.

The main branch muted tests are added to the muted tests for the branch (the file is not replaced, they are effectively combined). This means that an unmute has no effect.

• Use the ephemeral CI github token as the GH_TOKEN for gh
• For non-k8s steps in PR builds, fetch the latest mutes file and add it as an additional muted tests file
• If the muted-tests.yml file is changed as part of the PR, disable this functionality, otherwise a test unmute cannot be tested

Several scenarios were tested in various commits on this branch:

• Deleted the contents of muted-tests.yml and ensured all the mutes from main were pulled in
• Modified muted-tests.yml and ensured functionality was disabled

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[brianseeders]

💚 All backports created successfully

Status	Branch	Result
✅	8.x
✅	8.17
✅	8.16

Questions ?

Please refer to the Backport tool documentation