dev-forget-channel: accept passing in channel_id

This patch adds a channel_id parameter to allow for specifying
channels that are lacking a short_channel_id.

Useful in the case where a peer has 1) multiple channels (ONCHAIN etc)
and 2) a channel where the funding transaction hasn't been
broadcast/mined.

common/json_helpers.h

@@ -4,6 +4,7 @@
 #include "config.h"
 #include <bitcoin/tx.h>
 #include <common/json.h>
+#include <wire/wire.h>
 
 struct amount_msat;
 struct amount_sat;
@@ -44,6 +45,10 @@ bool json_to_msat(const char *buffer, const jsmntok_t *tok,
 bool json_to_txid(const char *buffer, const jsmntok_t *tok,
 		  struct bitcoin_txid *txid);
 
+/* Extract a channel id from this */
+bool json_to_channel_id(const char *buffer, const jsmntok_t *tok,
+			struct channel_id *cid);
+
 /* Split a json token into 2 tokens given a splitting character */
 bool split_tok(const char *buffer, const jsmntok_t *tok,
 				char split,