Use GitHub API to create pull request

[Leonidas-from-XIV]

Currently users need to have a clone of opam-repository locally available to create a new branch, add the generated opam files, commit, push and open a PR to opam-repository to get it merged.

The GitHub GraphQL API offers createCommitOnBranch and createPullRequest which should allow creating a PR without having to clone opam-repository. Paired with #409 this would mean that the user doesn't necessarily need to do anything in addition to have their changes included in opam-repository.

Of course, existing usecases with people with existing clones should remain supported, since that's also a common usecase, especially if changes are requested by OPAM Repository maintainers.

[NathanReb]

One thing worth noting is that we'll have to make sure we can create a branch on top of the target repository's default branch. I'm sure this will be possible but we must not forget about it!