feat: rewrite the nip-4e implementation (#1)

Make NIP-4e a core feature, not an optional feature.

Note:
- The UI is broken and needs to be updated in a separate PR.

Reviewed-on: #1

crates/relay_auth/Cargo.toml

@@ -17,4 +17,5 @@ nostr-sdk.workspace = true
 anyhow.workspace = true
 smallvec.workspace = true
 smol.workspace = true
+flume.workspace = true
 log.workspace = true