options: create enable/disable option for peer storage.

Since it's not spec-final yet (hell, it's not even properly specified
yet!) we need to put it behind an experimental flag.

Unfortunately, we don't have support for doing this in a plugin; a
plugin must present features before parsing options.  So we need to do
it in core.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-listconfigs.7.md

@@ -62,6 +62,7 @@ On success, an object is returned, containing:
 - **experimental-offers** (boolean, optional): `experimental-offers` field from config or cmdline, or default
 - **experimental-shutdown-wrong-funding** (boolean, optional): `experimental-shutdown-wrong-funding` field from config or cmdline, or default
 - **experimental-websocket-port** (u16, optional): `experimental-websocket-port` field from config or cmdline, or default
+- **experimental-peer-storage** (boolean, optional): `experimental-peer-storage` field from config or cmdline, or default *(added v23.02)*
 - **database-upgrade** (boolean, optional): `database-upgrade` field from config or cmdline
 - **rgb** (hex, optional): `rgb` field from config or cmdline, or default (always 6 characters)
 - **alias** (string, optional): `alias` field from config or cmdline, or default
@@ -223,4 +224,4 @@ RESOURCES
 
 Main web site: <https://github.com/ElementsProject/lightning>
 
-[comment]: # ( SHA256STAMP:581225b26efd84bfa99dc98e7a91e6fae11ef0b11939031d3da07f751f6d8f87)
+[comment]: # ( SHA256STAMP:1088401b9aeae1e079dab550d3b035ef82195f0466ad471bc7373386182f37dc)

doc/lightningd-config.5.md

@@ -714,6 +714,12 @@ connections on that port, on any IPv4 and IPv6 addresses you listen
 to ([bolt][bolt] #891).  The normal protocol is expected to be sent over WebSocket binary
 frames once the connection is upgraded.
 
+* **experimental-peer-storage**
+
+  Specifying this option means we will store up to 64k of encrypted
+data for our peers, and give them our (encrypted!) backup data to
+store as well, based on a protocol similar to [bolt][bolt] #881.
+
 BUGS
 ----
 

doc/schemas/listconfigs.schema.json

@@ -137,6 +137,11 @@
       "type": "u16",
       "description": "`experimental-websocket-port` field from config or cmdline, or default"
     },
+    "experimental-peer-storage": {
+      "type": "boolean",
+      "added": "v23.02",
+      "description": "`experimental-peer-storage` field from config or cmdline, or default"
+    },
     "database-upgrade": {
       "type": "boolean",
       "description": "`database-upgrade` field from config or cmdline"