Support for generating tokens for different servers

[cliffchapmanrbx]

We're currently working on a workflow that involves automating acceptance tests for a staging deployment of GitHub Enterprise. The tests are run from our primary GitHub Enterprise server while the staging server is not in a state to run GHA workflows itself yet.

We've determined you cannot override the env vars that are set automatically in a way that this Action would be able to detect.

Please add an input to allow for an arbitrary server URL to be used as the baseURL parameter, so we can selectively override it when necessary. It should likely still default to the GITHUB_API_URL env var.

Thank you for the very useful action!