Provide setting option for auth provider

[patrickpichler]

Feature Description

It would be nice if instead of having my gitlab token in either the env variable or properties file, I can specify a lua callback
to implement getting the password myself.

Something along the line of

    require("gitlab").setup({
      auth_provider = function()
        return "my_token", "https://custom.gitlab.instance.url"
      end,
    }

(Note: I have a PR ready, so if this is something you do not want in, feel free to decline it 😅 )

[harrisoncramer]

Addressed in #270, will be shipped with next release. Thank you Patrick for the contribution!