Make low-level Requester and GitHubClient APIs public

[Vampire]

For being able to use missing API functionality it would be nice if the Requester would be accessible, so that the low-level requests can be done manually or e. g. with Kotlin extension functions, but still having the lib do rate limiting and so on.

[bitwiseman]

@Vampire
Seems easy enough (public in a few places). I'm a little concerned though, as then then we'd be committed to the API of the Requester and supporting it.

[ranma2913]

This can be done in Groovy already, but isn't possible in Java. Probably not in Kotlin either:

GHRepository deleteSecret(GHRepository repo, String key) {
    // must use the raw request because kohsuke doesn't have a secret delete yet.
    repo.getRoot()
        .createRequest() // thank you Groovy for allowing this.
        .method("DELETE")
        .withUrlPath("/repos/${repo.getFullName()}/actions/secrets/${key}")
        .send()
    return repo
  }

[Vampire]

Of course, with Groovy you can even access private stuff easily. And you can do it in Java or Kotlin too with using reflection. But whether you should do it, is a different question, and a stable public API would be preferable