Add support for GitHub Enterprise

This gateway's v1-compatible predecessor was capable of interacting with GitHub Enterprise as well as GitHub.

If my recollection is correct, it should not be too difficult to add support for that here as well.

The one complication is probably that we don't have a GH Enterprise license that will easily allow us to develop or test all of that.

This isn't a high priority item, but if anyone wants to voluntarily tackle this, they are welcome to.