Add GitHub Enterprise Server support

[theztefan]

Changes to make this action work on GitHub Enterprise Server instances.

• Added gh_api_url as input parameter to the action
• Read the gh_api_url in and if defined set it as Octokit baseUrl. If not set it defaults to GHEC https://api.github.com
• Added feedback on the runs with few info messages
• Changed the action.yml to run the index.mjs instead of the old (?) index.js. ❗
  • deleted the old dist/index.js❗