chore: automate creation of release candidate branches

[awschristou]

This change builds on #1832 and #1827.

This change sets up a GitHub Action that will be used to create release candidate branches for agentic chat.

Once we have a commit on the main branch that we want to use as a release candidate (aka "preprod" internally) build, the new GitHub Action will be run, which creates a release candidate branch, and increments the agentic chat version. After the automation pushes the new commit into the new release candidate branch, the action from #1827 gets triggered, which produces the build. Then the action from #1832 gets triggered, which creates (or updates) a GitHub release.

Example, if agentic chat was versioned "1.2.3", and we initiate a release candidate using a minor version increment:

• the new version will become "1.3.0"
• a branch "release/agentic/1.3.0" will be created
• a commit will be pushed into that branch updating version.json (added in chore: create GitHub releases for agentic chat alpha, feature, and rc builds #1832)
• GitHub Actions will produce a GitHub release called "agentic-rc-1.3.0", which will contain downloadable builds
• the builds will be kept up to date with the latest commit from the release candidate branch

By using a separate staging branch for the release candidates, the main branch will no longer need to be locked while a release is in process.

To support this change:

• create-agent-standalone.yml was updated to get invoked on pushes to release/agentic/* branches
• a bug in create-agent-standalone.yml was fixed, so that it clones the branch/commit of interest (instead of the head of main)
• branch restrictions have been added to release/agentic/* branches
• a new secret RELEASE_CANDIDATE_BRANCH_CREATION_PAT was added to the repo, which is used by this Action, in order to ensure that our operations are able to trigger other GitHub actions. It uses the aws-toolkit-automation account to push the commit to the repo.

We won't use this automation until the internal pipelines have been set up to consume artifacts from the GitHub releases that these changes will produce.

Here's a quick sample of how the new Action will be run:

• you designate if the new agentic chat version is a (semver) major/minor/patch increment
  • alternatively, you can specify the full version, this is an escape hatch should a situation require it.
• you indicate what commit to seed the release candidate branch with

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.