Change Github API URL (support Gitea/Forgejo)

[mkody]

Hi!

I wanted to use this action for my Gitea/Forgejo instance. It started pretty well but failed when it tried to fetch comments.

::group::compare
    ...Storing image for indexing
    ✓ Image stored for indexing
    ...Indexing
    ✓ Indexed 1179 packages
    ✓ Provenance obtained from attestation
    ...Pulling
    ✓ Pulled
    ...Storing image for indexing
    ✓ Image stored for indexing
    ...Indexing
    ✓ Indexed 837 packages
    ...Pulling
    ✓ Pulled
::error::GET https://api.github.com/repos/kody/atw-watcher/issues/4/comments?direction=desc&per_page=10&sort=updated: 401 Bad credentials []

The GET would have worked if it was with my instance's domain and base path.

Would adding an input to specify the base API URL be possible?
It would be to replace https://api.github.com with https://<gitea domain>/api/v1.
The comments endpoints should work the same as GitHub's, but just in case here's Gitea's swagger from the demo instance.

This could also make it work with other GitHub API-compatible software.

EDIT: I see some actions use this in their inputs inside action.yml:

inputs:
  github-url:
    description: 'Base url of GitHub API'
    required: false
    default: ${{ github.api_url }}