Add rebase when PRs move from draft to ready for review

[itschrishudson]

Resolves https://github.com/brave/devops/issues/6595
Creates an action that will perform a rebase when a PR is moved from draft to ready for review.
This is to trigger a fresh CI run (as we don't run CI against draft PRs) and ensure the PR is up to date before being reviewed and merged.
If the label CI/run-draft is present, it will not rebase as CI will have been run on pushes to the branch.
The rebase label is added as part of the action to give visibility to PR owners and reviewers this has taken place.

Submitter Checklist:

• Confirms that no security/privacy review is needed, or that one has been requested
• There is a ticket for this
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• All relevant documentation has been updated - at least here
• Performed a self-review before removing WIP label
• Asked for review from the right people

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• Major classes, functions and non-trivial code blocks are well-commented
• Code follows the style guide
• Plans below are specified in PR before merging
• Branch being deployed from is protected in GitHub (PR approval required and no force push) - production only

Deploy Plan:

• N/A

Rollback Plan:

• N/A

Test Plan:

• N/A

This change has been deployed to:

• development
• staging
• production (tag @channel in #prod-changes if this may have wider impact)
• N/A