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/src/error.rs

@@ -29,6 +29,9 @@ pub enum Error {
     #[error("json error: {0}")]
     Json(#[from] serde_json::Error),
 
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
+
     #[error("nostrdb error: {0}")]
     Nostrdb(#[from] nostrdb::Error),