Advertise known nodes

• Adds a /api/v1/federation/domains that list known domains (except blocked ones). Endpoint is accessible to everyone, unless API Requires Authentication is enabled
• Add links to previous endpoint in nodeinfo payload when stats sharing is enabled

Context

This is helpful to map the network and to power future discovery features. For instance, if you now one node, you could discover other nodes using this mechanism.

cc @funkwhale/reviewers-python