splicing: Add channel state AWAITING_SPLICE

Update gossip routiens and various other hecks on the channel state to consider AWAITING_SPLICE to be routable and treated similar to CHANNELD_NORMAL.

Small updates to psbt interface

Changelog-None

common/jsonrpc_errors.h

@@ -65,6 +65,13 @@ enum jsonrpc_errcode {
 	FUNDING_STATE_INVALID = 312,
 	FUND_CANNOT_AFFORD_WITH_EMERGENCY = 313,
 
+	/* Splice errors */
+	SPLICE_BROADCAST_FAIL = 350,
+	SPLICE_WRONG_OWNER = 351,
+	SPLICE_UNKNOWN_CHANNEL = 352,
+	SPLICE_INVALID_CHANNEL_STATE = 353,
+	SPLICE_NOT_SUPPORTED = 354,
+
 	/* `connect` errors */
 	CONNECT_NO_KNOWN_ADDRESS = 400,
 	CONNECT_ALL_ADDRESSES_FAILED = 401,