support multiple destinations

[CrimsonFez]

We should support multiple destinations.
Destinations defined in 'extra_destinations' would take the base registry/repo settings and apply their values as overrides.
Values that can be overridden are:

• registry
• repo
• username
• password

settings:
  registry: harbor.404systems.net
  repo: 404systems/plugin-buildah
  cache_repo: 404systems/plugin-buildah/cache
  username: ...
  password: ...
  extra_destinations:
  - registry: ghcr.io
    username: ...
    password: ...
  - registry: docker.io
    repo: 404systems-net/plugin-buildah
    username: ...
    password: ...

I don't see a use case for supporting different tags per registry. Plus if we did support that we would have to consider the potential tag policy system.

Any comments or ideas on what this spec would look like are welcome :)