multi: add p2tr tapscript key path signing capabilities

lnrpc/signrpc/signer.swagger.json

@@ -325,7 +325,7 @@
         "witness_script": {
           "type": "string",
           "format": "byte",
-          "description": "The full script required to properly redeem the output.  This field will\nonly be populated if a p2wsh or a p2sh output is being signed."
+          "description": "The full script required to properly redeem the output. This field will\nonly be populated if a p2tr, p2wsh or a p2sh output is being signed. In case\ntaproot_key_spend is set to true then this value must correspond to the\ntaproot root hash (in case of a tapscript output) or the tap hashed internal\npublic key (in case of a BIP-0086 output)."
         },
         "output": {
           "$ref": "#/definitions/signrpcTxOut",
@@ -340,6 +340,10 @@
           "type": "integer",
           "format": "int32",
           "description": "The target input within the transaction that should be signed."
+        },
+        "taproot_key_spend": {
+          "type": "boolean",
+          "description": "Indicates that this should produce a signature that can be used for the key\nspend path of a taproot input. This requires the witness_script field to be\nset to the taproot root hash (in case of a tapscript output) or the tap\nhashed internal public key (in case of a BIP-0086 output)."
         }
       }
     },