Shell: add RPC to list network protocol versions.

src/node/shell/node_rpc.ml

@@ -459,6 +459,10 @@ let build_rpc_directory node =
     let implementation () =
       Node.RPC.Network.stat node |> RPC.Answer.return in
     RPC.register0 dir Services.Network.stat implementation in
+  let dir =
+    let implementation () =
+      RPC.Answer.return Distributed_db.Raw.supported_versions in
+    RPC.register0 dir Services.Network.versions implementation in
   let dir =
     let implementation () =
       let stream, stopper = Node.RPC.Network.watch node in

src/node/shell/node_rpc_services.ml

@@ -509,6 +509,13 @@ module Network = struct
       ~construct:Point.to_string
       ()
 
+  let versions =
+    RPC.service
+      ~description:"Supported network layer versions."
+      ~input: empty
+      ~output: (list P2p.Version.encoding)
+      RPC.Path.(root / "network" / "versions")
+
   let stat =
     RPC.service
       ~description:"Global network bandwidth statistics in B/s."

src/node/shell/node_rpc_services.mli

@@ -124,6 +124,9 @@ module Network : sig
   val stat :
     (unit, unit, unit, P2p.Stat.t) RPC.service
 
+  val versions :
+    (unit, unit, unit, P2p.Version.t list) RPC.service
+
   val events :
     (unit, unit, unit, P2p.RPC.Event.t) RPC.service