Adding gitspiegel-trigger workflow

[mutantcornholio]

Using a workflow to trigger mirroring instead of a webhook allows us to reuse "Approving workflow runs from public forks" GitHub feature to somewhat protect us from malicious PRs

[mutantcornholio]

UPD: The first attept to use a workflow to protect GitLab CI from untrusted contributors failed, because GitHub doesn't pass secrets to workflows for PRs that originate from forks.

This uses a different approach: instead of triggerring gitspiegel API directly from the workflow, we're just spawning an empty workflow with a specific path, and gitspiegel listens for workflow_run event to start mirroring.

The idea is the same: for the first-time contributors, running workflows would require manual aciton and that would block mirroring. But this time, we don't need any secrets to make it work.