Support installing plugins from GitHub Enterprise

[bendrucker]

TFLint plugins are installed from GitHub, using its releases API. While there are no plans to support other plugin sources, we should support alternate GitHub installations (GitHub Enterprise).

See BaseURL: https://pkg.go.dev/github.com/google/go-github/github#Client

Currently the code creates a new GitHub client for each installation rather than sharing across the whole command run. So it's theoretically possible to mix plugins from different hosts without major refactoring. Currently, the internal plugin config has SourceOwner and SourceRepo. It would also need SourceHost. That could then be used to construct the base URL for the GitHub client for that plugin.

Debug logs that hardcode https://api.github.com should be amended to use BaseURL.

Originally requested in #1202 (comment)

[ChiefAntiphates]

This is an issue that we've just encountered. We've written a custom plugin and created a release but are unable to use that plugin because the source is github.<company_name>.com as we are using a github enterprise.

Are there plans to implement this in the near future?

[bendrucker]

There are no plans, if you need this in the near future please take the time to follow the guidance above and make an attempt at a PR.

[bendrucker]

Re: #1734

If this could be altered to include a wildcard between github and .com then we could use our plugin hosted on our enterprise

Works for your setup, sure, but not an appropriate generic solution. There's no requirement on hostnames and users could pick gh, ghe, or literally anything else.

If anything, the way to validate a GitHub server is to make a request to / and look for either response headers (X-Github-Request-Id) or body keys (respository_url). Or the requests could just be allowed to fail later if an invalid host is supplied, no different than providing a non-existing org/repo.

If you can get this 80% of the way there, I can review/finish it. I don't have a GHES instance to test against nor can we test this in CI so we definitely need someone to step up to see this one through.