Allow pay command to specify amount even when it is part of the invoice

Implements possible solution 1 
as described in this [https://github.com/ElementsProject/lightning/issues/7020](this issue).

This should allow you to use the pay command with a rune that restricts the amount that can be spent, even when the invoice contains the amount.

plugins/pay.c

@@ -1183,9 +1183,9 @@ static struct command_result *json_pay(struct command *cmd,
 		return command_fail(cmd, PAY_INVOICE_EXPIRED, "Invoice expired");
 
 	if (invmsat) {
-		if (msat) {
+		if (msat && *msat != *invmsat) {
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-					    "msatoshi parameter unnecessary");
+					    "msatoshi parameter must be None or equal to value decoded from invoice");
 		}
 		p->amount = *invmsat;
 		tal_free(invmsat);