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.swagger.json

@@ -636,6 +636,11 @@
         "version": {
           "$ref": "#/definitions/signrpcMuSig2Version",
           "description": "The mandatory version of the MuSig2 BIP draft to use. This is necessary to\ndifferentiate between the changes that were made to the BIP while this\nexperimental RPC was already released. Some of those changes affect how the\ncombined key and nonces are created."
+        },
+        "pregenerated_local_nonce": {
+          "type": "string",
+          "format": "byte",
+          "description": "A set of pre generated secret local nonces to use in the musig2 session.\nThis field is optional. This can be useful for protocols that need to send\nnonces ahead of time before the set of signer keys are known. This value\nMUST be 97 bytes and be the concatenation of two CSPRNG generated 32 byte\nvalues and local public key used for signing as specified in the key_loc\nfield."
         }
       }
     },