Note multicasting

This is an initial implementation of note multicast, which sends posted
notes to other notedecks on the same network.

This came about after I nerd sniped myself thinking about p2p nostr on
local networks[1]

You can test this exclusively without joining any other relays by
passing -r multicast on the command line.

[1] https://damus.io/note1j50pseqwma38g3aqrsnhvld0m0ysdgppw6fjnvvcj0haeulgswgq80lpca

Signed-off-by: William Casarin <jb55@jb55.com>

crates/enostr/Cargo.toml

@@ -17,3 +17,5 @@ hex = { workspace = true }
 tracing = { workspace = true }
 thiserror = { workspace = true }
 url = { workspace = true }
+mio = { workspace = true }
+tokio = { workspace = true }