Added support for Enterprise GitHub

[yogeshkumararora]

No description provided.

[isaacsanders]

I think it might be better to use GH_HOST like the gh-cli does: https://cli.github.com/manual/gh_help_environment

[mheap]

I agree with @isaacsanders. I'd be happy to merge a GH_HOST PR

[marcellodesales]

I would suggest using process.env.GITHUB_API_URL, given that you are building a cloud-native solution... I will fork this repo and evaluate, but I would verify the following requirements:

IF your organization has a Github Actions mirror solution to GHES, then this action MUST:

   uses: org/repo@v1.0.0

• Query ${{ github.api_url }}/{owner}/{repo}/git/refs/v1.0.0 for the current server... If 404
  • Query https://api.github.com/repos/{owner}/{repo}/git/refs/v1.0.0 for the sake of verifying whether or not the action is actually from a verified Org owner, or/and has been a whitelisted Github Actions by the Enterprise Organization owner.
  • There's another tag value corner case from the Refs API I can't recall, but to make it fully work I think there's some verification and test cases to be added

At this point, the PR is fairly incomplete and it would break under our GHES and in future isolated GHEC, whose both ${{ github.api_url }} != https://api.github.com