Add a new repo provider

[starbelly]

In V7 the existing repo provider became organization which made room for a new repo provider. We should add a repo provider ala mix hex.repo.

See https://hexdocs.pm/hex/Mix.Tasks.Hex.Repo.html for an example.

The only question open I think is whether this overlaps with the would-be config provider. Specifically, will probably operate on a per repository basis vs globally, at least for a lot of options. For example, you wouldn't want to have rebar3 hex config api_url https://foo globally set, as it would effect all other repositories.

If the config provider operates on a per repository basis, then it may be that repo provider is somewhat pointless. The only exception to this would be the --fetch-public-key option that is available on mix hex.repo.