Add merge queue DCO workflow (#8086)
Add a GitHub Actions DCO check that runs for both `pull_request` and
`merge_group` events.
The checker is intentionally kept inline in the workflow for this
bootstrap change. If the logic lived in a separate repository script,
the workflow would need to check out that script before it exists on
`master`, so this first PR could fail against its own base branch.
Checking out helper code from the PR branch would also make the required
DCO gate depend on code supplied by the PR under test. After this
workflow lands on `master`, the helper can be moved into a separate
script in a follow-up cleanup.
Signed-off-by: Masahiro Tanaka <mtanaka@anyscale.com>
