json: add "json-to-psbt" helper

Pull a PSBT out of a json buffer

common/json_helpers.c

@@ -12,6 +12,7 @@
 #include <common/type_to_string.h>
 #include <common/wireaddr.h>
 #include <errno.h>
+#include <wally_psbt.h>
 
 bool json_to_bitcoin_amount(const char *buffer, const jsmntok_t *tok,
 			    uint64_t *satoshi)
@@ -127,6 +128,13 @@ bool json_to_preimage(const char *buffer, const jsmntok_t *tok, struct preimage
 	return hex_decode(buffer + tok->start, hexlen, preimage->r, sizeof(preimage->r));
 }
 
+bool json_to_psbt(const tal_t *ctx, const char *buffer,
+		  const jsmntok_t *tok, struct wally_psbt **dest)
+{
+	*dest = psbt_from_b64(ctx, buffer + tok->start, tok->end - tok->start);
+	return dest != NULL;
+}
+
 void json_add_node_id(struct json_stream *response,
 		      const char *fieldname,
 		      const struct node_id *id)

common/json_helpers.h

@@ -17,6 +17,7 @@ struct secret;
 struct short_channel_id;
 struct wireaddr;
 struct wireaddr_internal;
+struct wally_psbt;
 
 /* Decode a hex-encoded payment preimage */
 bool json_to_preimage(const char *buffer, const jsmntok_t *tok, struct preimage *preimage);
@@ -24,6 +25,10 @@ bool json_to_preimage(const char *buffer, const jsmntok_t *tok, struct preimage
 /* Extract a secret from this. */
 bool json_to_secret(const char *buffer, const jsmntok_t *tok, struct secret *dest);
 
+/* Extract a psbt from this. */
+bool json_to_psbt(const tal_t *ctx, const char *buffer,
+		  const jsmntok_t *tok, struct wally_psbt **dest);
+
 /* Extract a pubkey from this */
 bool json_to_pubkey(const char *buffer, const jsmntok_t *tok,
 		    struct pubkey *pubkey);