Investigate replacing local docker relays with MockRelay

[erskingardner]

The rust nostr creates now include a relay builder crate that has a MockRelay and LocalRelay that might be useful for us. It might allow us to stop running local relays in a docker container for development. We should explore this and see if it's going to be useful/worthwhile.

https://github.com/rust-nostr/nostr/tree/master/crates/nostr-relay-builder

One important thing to note here is that we would need to make sure that MockRelay or LocalRelay continues to work with our Flutter development environment. So we'd still need to be able to run the app on an emulator and phone.

[erskingardner]

it might also be that the local/mock relay is really helpful for tests only but we keep running local relays for development for data persistence across builds and local testing.