repos: Add err-githubhook.
Merge pull request #290 from daenney/err-githubhook

repos: Add err-githubhook.

Thanks for the contribution !
streaming core part 1

we have the stream callback contract
and a Tee that can stream from one source to multiple destinations
First hook on tox backend

It works, still have very rough edges.
implemented outgoing transfers

With a reference implementation with the tox backend.