lnrpc: add new locked balance field for WalletBalance

In this commit, we add a new field to the WalletBalance call that
permits users to account for the set of outputs that may be locked due
to a pending transaction. Without this field any time users locked
outputs for things like PSBT signing, then they disappear from the
WalletBalance call, which may cause a panic.

lnrpc/lightning.proto

@@ -2502,6 +2502,10 @@ message WalletBalanceResponse {
     // The unconfirmed balance of a wallet(with 0 confirmations)
     int64 unconfirmed_balance = 3;
 
+    // The total amount of wallet UTXOs held in outputs that are locked for
+    // other usage.
+    int64 locked_balance = 5;
+
     // A mapping of each wallet account's name to its balance.
     map<string, WalletAccountBalance> account_balance = 4;
 }

lnrpc/lightning.swagger.json

@@ -6540,6 +6540,11 @@
           "format": "int64",
           "title": "The unconfirmed balance of a wallet(with 0 confirmations)"
         },
+        "locked_balance": {
+          "type": "string",
+          "format": "int64",
+          "description": "The total amount of wallet UTXOs held in outputs that are locked for\nother usage."
+        },
         "account_balance": {
           "type": "object",
           "additionalProperties": {