walletrpc+itest: add SignPsbt RPC

lnrpc/walletrpc/walletkit.swagger.json

@@ -364,6 +364,40 @@
         ]
       }
     },
+    "/v2/wallet/psbt/sign": {
+      "post": {
+        "summary": "SignPsbt expects a partial transaction with all inputs and outputs fully\ndeclared and tries to sign all unsigned inputs that have all required fields\n(UTXO information, BIP32 derivation information, witness or sig scripts)\nset.\nIf no error is returned, the PSBT is ready to be given to the next signer or\nto be finalized if lnd was the last signer.",
+        "description": "NOTE: This RPC only signs inputs (and only those it can sign), it does not\nperform any other tasks (such as coin selection, UTXO locking or\ninput/output/fee value validation, PSBT finalization). Any input that is\nincomplete will be skipped.",
+        "operationId": "WalletKit_SignPsbt",
+        "responses": {
+          "200": {
+            "description": "A successful response.",
+            "schema": {
+              "$ref": "#/definitions/walletrpcSignPsbtResponse"
+            }
+          },
+          "default": {
+            "description": "An unexpected error response.",
+            "schema": {
+              "$ref": "#/definitions/rpcStatus"
+            }
+          }
+        },
+        "parameters": [
+          {
+            "name": "body",
+            "in": "body",
+            "required": true,
+            "schema": {
+              "$ref": "#/definitions/walletrpcSignPsbtRequest"
+            }
+          }
+        ],
+        "tags": [
+          "WalletKit"
+        ]
+      }
+    },
     "/v2/wallet/send": {
       "post": {
         "summary": "SendOutputs is similar to the existing sendmany call in Bitcoind, and\nallows the caller to create a transaction that sends to several outputs at\nonce. This is ideal when wanting to batch create a set of transactions.",
@@ -1392,6 +1426,26 @@
         }
       }
     },
+    "walletrpcSignPsbtRequest": {
+      "type": "object",
+      "properties": {
+        "funded_psbt": {
+          "type": "string",
+          "format": "byte",
+          "description": "The PSBT that should be signed. The PSBT must contain all required inputs,\noutputs, UTXO data and custom fields required to identify the signing key."
+        }
+      }
+    },
+    "walletrpcSignPsbtResponse": {
+      "type": "object",
+      "properties": {
+        "signed_psbt": {
+          "type": "string",
+          "format": "byte",
+          "description": "The signed transaction in PSBT format."
+        }
+      }
+    },
     "walletrpcTransaction": {
       "type": "object",
       "properties": {