build: auto start Jenkins CI via PR labels

Add an Action that will find every PR with the `request-ci` label and
will start a Jenkins CI for each of these Pull Requests. The scheduler
event is used to circumvent GitHub Actions limitations on Pull Requests
from forks (where secrets are not accessible and the GITHUB_TOKEN is
read-only).

If the Action fails to start a CI, it will add a `request-ci-failed`
label and will leave a comment with the error message from NCU.

Fixes: https://github.com/nodejs/github-bot/issues/234

PR-URL: https://github.com/nodejs/node/pull/34089


Reviewed-By: Christian Clauss <cclauss@me.com>