Use Github API to create a fork of opam-repository #409
Description
Currently the setup of dune-release requires users to fork opam-repository manually and letting dune-release know about it.
This is a very transparent process and many power user already have their forks created in any case. Yet for people new to the ecosystem telling them to do all these extra step is an additional burden. Fortunately the GitHub GraphQL API provides the cloneProject mutation which should make it possible to fork opam-repository into the users own repository automatically.
This should of course only happen if the user doesn't already have a fork or specified a repository explicitly.