Forward api calls for ruler for enabling Grafana unified alerting

[goelankitt]

This PR adds routes for redirecting api calls to ruler for enabling grafana unified alerting. This is needed because at this point grafana does not allow to specify a separate url for ruler.

https://github.com/grafana/grafana/pull/41528/files

[goelankitt]

@weeco Thoughts about this?

[weeco]

Hey @goelankitt , please be aware that I'm no longer working at REWE digital and therefore I will not be able to maintain it. I'm unsure whether REWE digital plans to maintain that gateway or not. Maybe @elmarx can help to clarify and provide a more definitive answer.

However this project is very simple and I think forking it is not unreasonable.

[goelankitt]

Thanks @weeco for the reply. I will go ahead and fork it :)