Populate additional sources based on available Vendor kind

[mszostok]

Description

As the #560 was done. We can now iterate with a new feature which will make use of Vendor kind.

It's a part of more complex issue about federation: #511

Reason

Currently, information about external Hub needs to be described in readme files under set up section. For example, you need to describe that to populate manifests from google/hub-manifests which depends on capactio/hub-manifests such command needs to be executed:

populator register ocf-manifests --source github.com/google/hub-manifests --source github.com/capactio/hub-manifests

If you forget about --source github.com/capactio/hub-manifests it will not work properly. Instead of doing that, we can describe this relation inside google/hub-manifests repo using Vendor kind. CLI should automatically discover all Vendor manifests and based on them populate data from all additional sources.

Use cases

• Content Developer - base on the Vendor kinds, knows which hubs are available. As a result, can consume already existing content like types, or provide new Implementation for already existing Interface.
• System User - doesn't need to follow a special instruction for each Hub to set it up properly.

Out of scope

• handle private Hubs
• caching
• support different sources, not only git or Public Hub API. We can try to reuse the same concept developed for the delegated storage.
• content synchronization (periodic/hooks). Make it configurable.