Feat (dex): intake new homeservers

[SHAcollision]

We need to track new homeservers automatically. As well as disregard malicious ones.

Options:

1. Direct homeserver submission POST endpoint where new send their homeserver pubky to Nexus on sign up.
2. Crawl new users' homeservers by reading and parsing URIs and resolving them from existing homeservers.

Issue of 1: leads to centralisation of indexers, users will only submit to a few indexers at best. This could be alleviated if Nexus also freely exposes the curated list of homeservers via public API and can read from other Nexus and intake homeservers that way.

Issue of 2: chicken and egg problem, ho is going to mention or publish a URI of user that has not yet been indexed? Maybe this could be a new service? Publisher?