Expose an endpoint to trigger a repository fetch

[nlewo]

In order to avoid wating for the next trigger, it would be nice to add a way allowing users/CIs to trigger a fetch.

(Note this has been actually already drafted but removed because i didn't use it.)

[colemickens]

Additionally, if you use the right headers (E-Tag, I think), you can poll GitHub somewhat rapidly, without exhausting quota. It's a nice option for those who might not have permissions to admin webhooks.

[devzero]

I'm looking at implementing something using websocat or sockethook so that you can listen to a forge's webhook and expose that as a websocket that machines that can't expose ports can still get near-realtime triggering without having to poll a server