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.

lnrpc/signrpc/signer.proto

@@ -558,6 +558,16 @@ message MuSig2SessionRequest {
     combined key and nonces are created.
     */
     MuSig2Version version = 6;
+
+    /*
+    A set of pre generated secret local nonces to use in the musig2 session.
+    This field is optional. This can be useful for protocols that need to send
+    nonces ahead of time before the set of signer keys are known. This value
+    MUST be 97 bytes and be the concatenation of two CSPRNG generated 32 byte
+    values and local public key used for signing as specified in the key_loc
+    field.
+    */
+    bytes pregenerated_local_nonce = 7;
 }
 
 message MuSig2SessionResponse {
@@ -684,4 +694,4 @@ message MuSig2CleanupRequest {
 }
 
 message MuSig2CleanupResponse {
-}
+}