Allow a DNSResolverMessageHandler to set dns_resolver feature

A `DNSResolverMessageHandler` which handles resolution requests
should want the `NodeFeatures` included in the node's
`node_announcement` to include `dns_resolver` to indicate to the
world that it provides that service. Here we enable this by
requesting extra feature flags from the `DNSResolverMessageHandler`
in the features `OnionMessenger`, in turn, provides to
`PeerManager` (which builds the `node_announcement`).

lightning/src/onion_message/dns_resolution.rs

@@ -34,6 +34,8 @@ use dnssec_prover::validation::verify_rr_stream;
 
 use dnssec_prover::rr::Name;
 
+use lightning_types::features::NodeFeatures;
+
 use crate::blinded_path::message::DNSResolverContext;
 use crate::io;
 #[cfg(feature = "dnssec")]
@@ -67,6 +69,13 @@ pub trait DNSResolverMessageHandler {
 	/// With this, we should be able to validate the DNS record we requested.
 	fn handle_dnssec_proof(&self, message: DNSSECProof, context: DNSResolverContext);
 
+	/// Gets the node feature flags which this handler itself supports. Useful for setting the
+	/// `dns_resolver` flag if this handler supports returning [`DNSSECProof`] messages in response
+	/// to [`DNSSECQuery`] messages.
+	fn provided_node_features(&self) -> NodeFeatures {
+		NodeFeatures::empty()
+	}
+
 	/// Release any [`DNSResolverMessage`]s that need to be sent.
 	fn release_pending_messages(&self) -> Vec<(DNSResolverMessage, MessageSendInstructions)> {
 		vec![]

lightning/src/onion_message/messenger.rs

@@ -1778,7 +1778,7 @@ where
 	fn provided_node_features(&self) -> NodeFeatures {
 		let mut features = NodeFeatures::empty();
 		features.set_onion_messages_optional();
-		features
+		features | self.dns_resolver_handler.provided_node_features()
 	}
 
 	fn provided_init_features(&self, _their_node_id: PublicKey) -> InitFeatures {