KAFKA-18792 Add workflow to check PR format

[mumrah]

Adds two new workflows to help us enforce some uniform PR structure. The first workflow runs in an unprivileged context and simply captures the PR number into a text file and archives it. This is then used by another workflow that runs in a privileged context using the code in trunk to actually do the validation.

The validation is done using a new Python script. This script fetches a PR using the GitHub CLI and validates its structure. For now this just includes the title and body, but could perform other non-code related checks in the future.

This validation is needed for the up-coming merge queue functionality.

[mumrah]

To test this PR:

• Install gh if you don't have it already (brew install gh)
• Checkout this PR: gh pr checkout 18985
• Run the script against any PR

GITHUB_ACTIONS=1 PR_NUMBER=18955 python .github/scripts/pr-format.py

• Observe success or failure. Note that basically all PRs will fail the new required "Reviewers:" line in the PR body.

We cannot test the full automation until this PR is merged since part of the workflow runs off of trunk.

[jolshan]

I ran the command on this PR and it seems to have passed. Is that expected?

GITHUB_ACTIONS=1 PR_NUMBER=18985 python .github/scripts/pr-format.py
Read env GITHUB_ACTIONS: 1
Read env PR_NUMBER: 18985
PR format looks good!