input+signrpc+lnwallet+multi: implement combined nonce support

Add CombinedNonce() and RegisterCombinedNonce() methods with full implementation
stack.

Interface and core implementation:
- input/musig2.go: Added methods to MuSig2Session and MuSig2Signer interfaces
- input/musig2_session_manager.go: MusigSessionManager implementation using
  HaveAllNonces flag for state tracking (simplified, no extra fields)
- internal/musig2v040: Stub implementations returning ErrUnsupportedMethod
- Mock implementations (MockInputSigner, MockSigner, DummySigner)

RPC layer:
- lnrpc/signrpc/signer.proto: RPC method definitions and messages
- lnrpc/signrpc/signer.yaml: REST API endpoint mappings
- Generated protobuf code (all .pb.go files)
- lnwallet/rpcwallet/rpcwallet.go: RPCKeyRing client implementation

The proto types and RPCKeyRing are added together since RPCKeyRing implements
the Signer interface and requires proto types to fulfill the contract.

For v0.4.0, these methods return ErrUnsupportedMethod. Use MuSig2Version100RC2
to access these features.

watchtower/wtmock/signer.go

@@ -150,6 +150,22 @@ func (s *MockSigner) MuSig2RegisterNonces(input.MuSig2SessionID,
 	return false, nil
 }
 
+// MuSig2RegisterCombinedNonce registers a pre-aggregated combined nonce for a
+// session identified by its ID.
+func (s *MockSigner) MuSig2RegisterCombinedNonce(input.MuSig2SessionID,
+	[musig2.PubNonceSize]byte) error {
+
+	return nil
+}
+
+// MuSig2GetCombinedNonce retrieves the combined nonce for a session identified
+// by its ID.
+func (s *MockSigner) MuSig2GetCombinedNonce(input.MuSig2SessionID) (
+	[musig2.PubNonceSize]byte, error) {
+
+	return [musig2.PubNonceSize]byte{}, nil
+}
+
 // MuSig2Sign creates a partial signature using the local signing key
 // that was specified when the session was created. This can only be
 // called when all public nonces of all participants are known and have