Add GHA workflow PR Review Timeline

[shreyas-s-rao]

How to categorize this PR?

/area dev-productivity
/kind enhancement

What this PR does / why we need it:

Introduce new GHA workflow PR Review Timeline, which uses https://github.com/marketplace/actions/pr-review-timeline to publish the review timeline for a PR in the Checks tab. This makes it easier to track PR timelines, especially useful for large or long-running PRs with multiple review cycles.

Checklist:
NA

Special notes for your reviewer:

This is what the output will look like:

Current summary for this action is already available for this PR here: https://github.com/gardener/etcd-wrapper/actions/runs/21740476540?pr=79

Please note, this will only be available in the action execution page and not the PR description itself, since the action input publish-to-pr is set to false by default.

Release note:

Add GHA workflow `PR Review Timeline` for easy tracking of reviewer timelines for PRs.

[gardener-prow]

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