Add support for Hub federation

[mszostok]

Description

The idea is to enable support for connecting multiple Hubs into a single one.

Currently, we have an entity called vendor https://github.com/capactio/capact/blob/main/ocf-spec/0.0.1/README.md#vendor
Remote Public Hub repositories can be mounted under the vendor sub-tree in the local repository.
Vendor manifest stores connection details of the external OCH, such as URI of the repository (base path) or federation strategy.

This concept needs to be refined.

Features:

• support private hubs
• authorization/authentication
  • Signature-based
  • Take into account future plans: external IDPs - makes sense to use ORY stack with multiple authenticators
• 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.

In the first round, we should think how to enable Public Hub federation. In the future, the Local Hubs could also reuse the same implementation.

Reason

In the current approach, all content that is loaded to Public Hub requires to be defined in a single repository. There is no easy way to split it e.g. extract test content to separate och repository which can be optionally enabled.

This pattern doesn't scale well. We should be able to split repository per vendor (e.g. Google, Microsoft, RedHat etc.) and per use-cases e.g. private used for commercial use-cases, incubator, open-source manifests etc.

[mszostok]

Simplified approach for current use cases:

• Support populating Hub manifests from multiple sources #560
• Populate additional sources based on available Vendor kind #598