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.
Reviewers: Justine Olshan <jolshan@confluent.io>
David Arthur