taprpc: support custom lock ID and expiration in CommitVirtualPsbts

This change extends the CommitVirtualPsbts RPC to accept an optional
custom UTXO lock ID and lease expiration. By enabling callers to define
these parameters explicitly, the wallet can better coordinate UTXO
locking across concurrent workflows and provide safer, more flexible
transaction retargeting when using virtual PSBTs.

If unset, the default internal lock ID and 10-minute expiration remain
in effect, preserving backward compatibility.

Author: bhandras

rpcserver.go

@@ -2481,8 +2481,10 @@ func (r *rpcServer) CommitVirtualPsbts(ctx context.Context,
 		Template: &walletrpc.FundPsbtRequest_CoinSelect{
 			CoinSelect: coinSelect,
 		},
-		MinConfs:   1,
-		ChangeType: P2TRChangeType,
+		MinConfs:              1,
+		ChangeType:            P2TRChangeType,
+		CustomLockId:          req.CustomLockId,
+		LockExpirationSeconds: req.LockExpirationSeconds,
 	}
 
 	// Unfortunately we can't use the same RPC types, so we have to do a