plugins/topology: add direction field to listchannels.

It's a core concept in the spec which isn't directly exposed.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `listchannels` added a `direction` field (0 or 1) as per gossip specification.

doc/lightning-listchannels.7.md

@@ -36,6 +36,7 @@ On success, an object containing **channels** is returned.  It is an array of ob
 - **source** (pubkey): the source node
 - **destination** (pubkey): the destination node
 - **short\_channel\_id** (short\_channel\_id): short channel id of channel
+- **direction** (u32): direction (0 if source < destination, 1 otherwise).
 - **public** (boolean): true if this is announced (otherwise it must be our channel)
 - **amount\_msat** (msat): the total capacity of this channel (always a whole number of satoshis)
 - **message\_flags** (u8): as defined by BOLT #7
@@ -79,4 +80,4 @@ Lightning RFC site
 -   BOLT \#7:
     <https://github.com/lightning/bolts/blob/master/07-routing-gossip.md>
 
-[comment]: # ( SHA256STAMP:d8d52272963a9ec4708fd3ae41585ddd8120bb5444c03219a7b728f0b2e09ec3)
+[comment]: # ( SHA256STAMP:78f59780528ae5cd33c3607ed11b128cc94e1e0a5e2babd59cb99574a3f5f956)

doc/schemas/listchannels.schema.json

@@ -15,6 +15,7 @@
           "source",
           "destination",
           "short_channel_id",
+          "direction",
           "public",
           "amount_msat",
           "message_flags",
@@ -40,6 +41,10 @@
             "type": "short_channel_id",
             "description": "short channel id of channel"
           },
+          "direction": {
+            "type": "u32",
+            "description": "direction (0 if source < destination, 1 otherwise)."
+          },
           "public": {
             "type": "boolean",
             "description": "true if this is announced (otherwise it must be our channel)"