multi: add p2tr tapscript signing capabilities

lnrpc/signrpc/signer.proto

@@ -158,6 +158,12 @@ message SignReq {
 
     // A set of sign descriptors, for each input to be signed.
     repeated SignDescriptor sign_descs = 2;
+
+    /*
+    The full list of UTXO information for each of the inputs being spent. This
+    is required when spending one or more taproot (SegWit v1) outputs.
+    */
+    repeated TxOut prev_outputs = 3;
 }
 
 message SignResp {