json: Add two param functions to parse string arrs and outpoint arrs

In a couple of places we accept arrays of strings and don't validate
them. If we forward them, e.g., call a JSON-RPC method from the
plugin, we end up embedding the unverified string in the JSON-RPC
call without escaping, which then leads to invalid JSON being passed
on.

This at least partially causes #4238

bitcoin/tx.h

@@ -21,6 +21,12 @@ struct wally_psbt;
 struct bitcoin_txid {
 	struct sha256_double shad;
 };
+
+struct bitcoin_outpoint {
+	struct bitcoin_txid txid;
+	u16 n;
+};
+
 /* Define bitcoin_txid_eq */
 STRUCTEQ_DEF(bitcoin_txid, 0, shad.sha.u);