Add the ability to run CI on a different repository

[Kobzol]

For rust-lang/rust, we need the ability to push commits and run the CI on a different repository than where the final merge happens. So bors should be taught to change the actual repository into which it force-pushes the merges and where it reads the webhooks with test workflow results.

The name of the "alternate" repository should be read from the repository configuration.

[klaus82]

Hi! I would like to take care of this task.
I talked on LinkedIn with @MarcoIeni, and he points me to 'good first issue'.
I'm starting to understand the code but any suggestion is wellcome :)

[Kobzol]

Hi! Thanks for taking a look. I think that @vohoanglong0107 is already working on this, but I don't know the current status.