walletrpc+itest: add SignPsbt RPC

lnrpc/walletrpc/walletkit.proto

@@ -200,6 +200,21 @@ service WalletKit {
     */
     rpc FundPsbt (FundPsbtRequest) returns (FundPsbtResponse);
 
+    /*
+    SignPsbt expects a partial transaction with all inputs and outputs fully
+    declared and tries to sign all unsigned inputs that have all required fields
+    (UTXO information, BIP32 derivation information, witness or sig scripts)
+    set.
+    If no error is returned, the PSBT is ready to be given to the next signer or
+    to be finalized if lnd was the last signer.
+
+    NOTE: This RPC only signs inputs (and only those it can sign), it does not
+    perform any other tasks (such as coin selection, UTXO locking or
+    input/output/fee value validation, PSBT finalization). Any input that is
+    incomplete will be skipped.
+    */
+    rpc SignPsbt (SignPsbtRequest) returns (SignPsbtResponse);
+
     /*
     FinalizePsbt expects a partial transaction with all inputs and outputs fully
     declared and tries to sign all inputs that belong to the wallet. Lnd must be
@@ -820,6 +835,19 @@ message UtxoLease {
     uint64 expiration = 3;
 }
 
+message SignPsbtRequest {
+    /*
+    The PSBT that should be signed. The PSBT must contain all required inputs,
+    outputs, UTXO data and custom fields required to identify the signing key.
+    */
+    bytes funded_psbt = 1;
+}
+
+message SignPsbtResponse {
+    // The signed transaction in PSBT format.
+    bytes signed_psbt = 1;
+}
+
 message FinalizePsbtRequest {
     /*
     A PSBT that should be signed and finalized. The PSBT must contain all