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.

internal/musig2v040/context.go

@@ -59,6 +59,11 @@ var (
 	// ErrNotEnoughSigners is returned if a caller attempts to obtain an
 	// early nonce when it wasn't specified
 	ErrNoEarlyNonce = fmt.Errorf("no early nonce available")
+
+	// ErrUnsupportedMethod is returned when calling methods that are not
+	// supported in the legacy v0.4.0 implementation.
+	ErrUnsupportedMethod = fmt.Errorf("method not supported in MuSig2 " +
+		"v0.4.0")
 )
 
 // Context is a managed signing context for musig2. It takes care of things
@@ -668,3 +673,15 @@ func (s *Session) CombineSig(sig *PartialSignature) (bool, error) {
 func (s *Session) FinalSig() *schnorr.Signature {
 	return s.finalSig
 }
+
+// CombinedNonce is not supported in the legacy v0.4.0 implementation and will
+// always return an error.
+func (s *Session) CombinedNonce() ([PubNonceSize]byte, error) {
+	return [PubNonceSize]byte{}, ErrUnsupportedMethod
+}
+
+// RegisterCombinedNonce is not supported in the legacy v0.4.0 implementation
+// and will always return an error.
+func (s *Session) RegisterCombinedNonce(_ [PubNonceSize]byte) error {
+	return ErrUnsupportedMethod
+}