lnrpc/signrpc+lnwallet/rpcwallet: expose local nonces over RPC

In this commit, we update the set of protos to accept the local secret
nonces over RPC. This is actually a 97 byte value, as it includes the
two 32 byte nonces, as well as the 33 byte value of the public key of
the signer.

This is needed in order to be able to open taproot channels over the RPC
interface.

lnwallet/rpcwallet/rpcwallet.go

@@ -664,8 +664,6 @@ func (r *RPCKeyRing) MuSig2CreateSession(bipVersion input.MuSig2Version,
 		return nil, err
 	}
 
-	// TODO(roasbeef): add protos to specify session options
-
 	// We need to serialize all data for the RPC call. We can do that by
 	// putting everything directly into the request struct.
 	req := &signrpc.MuSig2SessionRequest{
@@ -708,6 +706,10 @@ func (r *RPCKeyRing) MuSig2CreateSession(bipVersion input.MuSig2Version,
 		}
 	}
 
+	if localNonces != nil {
+		req.PregeneratedLocalNonce = localNonces.SecNonce[:]
+	}
+
 	ctxt, cancel := context.WithTimeout(context.Background(), r.rpcTimeout)
 	defer cancel()