[coreapi] [feature] Add repository initialization support

The `doInit` function (used by `ipfs init`) is not exported and right now the closest functionality to that is the exported `fsrepo.Init` but that requires taking care of the `config.Config` structure (and other maintenance issues taken care of by `doInit`).

My use case is doing benchmark tests between `flatfs` and `badger` datastores where I create a repo from scratch to measure put/get times. For me the easiest procedure at the moment is calling the `ipfs init` shell command directly (with `exec.Command`) instead of using the internal API.