[damencho]

Adds the CLA check, which replaces the CLA reminder of the switched-off Jenkins ghprb PR jobs. It is the same caller as jitsi/jitsi-meet#18089 and runs the reusable workflow from jitsi/.github#3, pinned by SHA.

• Runs on pull_request_target (opened, reopened, synchronize), only in the jitsi org, so forks get no runs. PR code is never checked out.
• Org members, outside collaborators and bots are skipped.
• For everyone else, every commit author email of the PR is looked up in the hashed signer list in the private jitsi/cla-signers repo.
  • All found: the check passes and the PR gets info:cla-signed.
  • Otherwise: the check fails, the PR gets info:cla-missing and, once, the CLA comment with the ICLA/CCLA links.
• New signatures re-run the failed checks of open info:cla-missing PRs.

The status check is cla / CLA. It is not required yet. Because of pull_request_target, it only starts running on PRs once this is merged.