Introduce public API to retrieve an API key for organizations, given an authenticated user

[zachdaniel]

Use case

We're working on support for private packages with igniter (not external repos, that will come later), and part of the workflow includes looking up package information using the API. The user might issue this command:

mix igniter.install my_org/package

We would then want to read the package information via the API, and find out what the latest, non-revoked, non-release-candidate version is (or just latest non-revoked if nothing meets that criteria), and provide a "good" pin for it, i.e "~> 3.5" in the users mix.exs.

Current Work

Our first attempt was to use Mix.State.fetch!(:repos), but after discussion with @ericmj it became clear that the auth_key we get from that is not correct/sufficient for reading packages via the API. This code is in main of igniter presently. It works, in fact, for explicitly provided versions like mix igniter.install org/package@3.5, so all that is left for us to solve is:

1. have them run mix local.hex to update and get this new API.
2. get an api key for looking this info up (or tell the user to run mix hex.user auth and confirm they have access to the org

[zachdaniel]

For now, we can handle not having this key relatively easily w/o blocking the ability to install private packages:

        raise """
        Cannot currently determine the version for private packages automatically.
        
        Instead of `mix igniter.install #{org}/#{package}`, please include a version, i.e

            mix igniter.install #{org}/#{package}@2.4

        Replacing the version with the latest major/minor version.
        """

So this is not mega urgent, but would be great to figure out to make it easier to install private packages in igniter 😄

[ericmj]

Currently there is no public API so there are some things to discuss and decide on.

1. Where should the API live?
2. What backwards compatibility should be guaranteed?
3. How do you check if Hex is installed? Mix.Hex.ensure_installed?/1 is private https://github.com/elixir-lang/elixir/blob/81ef21272d9040b94eb8ff8e6965d85727c35a42/lib/mix/lib/mix/hex.ex#L17.
4. Depending on the backwards compatilibility there might be need to check the version also.
5. Are there any functions you need to call first, such as Hex.start/0?

[ericmj]

Also see: #1060