Add Support for GitHub Enterprise Instances

Hi, I wanted to use this action on an on premises GitHub Enterprise Server that has it’s own url (e.g. https://git.acme.com)

I was hoping it would be possible to add a new input to the action.yml to support passing in a url for a Git instance that would default to GitHub.com if unfilled.

Right now it looks like the message field will use the instance url for it’s link but the others do not.

https://github.com/8398a7/action-slack/blob/9c613f82be50ba1c5d845d3448861aba03da3d0b/src/fields.ts#L140

There might be a way to do it dynamically because this is how the message field gets the url for the link https://github.com/8398a7/action-slack/blob/9c613f82be50ba1c5d845d3448861aba03da3d0b/src/fields.ts#L73-L81

[8398a7]

We have released this feature in v3.9.0.

[8398a7]

@ruberVulpes I do not have the environment to try enterprise. Can I have you review the modifications for me? Imported features into the pre branch.

I think it can be executed as follows.

steps:
  - uses: 8398a7/action-slack@pre
    with:
      gitHubBaseUrl: https://git.acme.com
      status: ${{ job.status }}
      fields: repo,message,commit,author,action,eventName,ref,workflow,job,took
    env:
      GITHUB_API_URL: https://git.acme.com
      SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}

The following are the changes

• Add gitHubBaseUrl
• Add GITHUB_API_URL
• Using the pre branch

What happens if the environment variable for GITHUB_API_URL is enterprise? Will it be https://api.git.acme.com?