feat: GHES support

[trewallier]

Feature request: GitHub Enterprise Server support

It is not possible to set the GH_HOST environment variable for gh cli commands used by the implementation, making the action incompatible with self-hosted GHES. (cli manual for reference)

It would be nice to have an optional gh-host input variable to ensure compatibility. I am willing to implement and test this change if it fits your action @fgrosse.

[fgrosse]

Sure, go for it 😃