ci: Add pull request assigner workflow

This commit adds the "Pull Request Assigner" workflow that
automatically assigns the reviewers, assignee and labels for a pull
request.

Note that this workflow runs on the `pull_request_target` event in the
context of the base branch using the `MAINTAINERS.yml` from the base
branch.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>