[Github App] Fetch from remote URL on the same organization: suport GHE raw URL

[chmouel]

As mentioned here

https://github.com/openshift-pipelines/pipelines-as-code/blob/cfdd8f2331d12bfd0e3cf6028de993f24b3197fc/pkg/provider/github/github.go#L23-L28

We are only supporting public raw github url, we need to handle it in GHE.

I believe detecting if it's a raw.${GHE_URL} in this condition https://github.com/openshift-pipelines/pipelines-as-code/blob/cfdd8f2331d12bfd0e3cf6028de993f24b3197fc/pkg/provider/github/github.go#L48 shoudl be enough....