Start factoring out the serve protocol for Hydra to share

Factor out `ServeProto::BasicClientConnection` for Hydra to share

- `queryValidPaths`: Hydra uses the lock argument differently than Nix,
  so we un-hard-code it.

- `buildDerivationRequest`: Just the request half, as Hydra does some
  things between requesting and responding.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libstore/serve-protocol.hh

@@ -59,6 +59,13 @@ struct ServeProto
         Version version;
     };
 
+    /**
+     * Stripped down serialization logic suitable for sharing with Hydra.
+     *
+     * @todo remove once Hydra uses Store abstraction consistently.
+     */
+    struct BasicClientConnection;
+
     /**
      * Data type for canonical pairs of serialisers for the serve protocol.
      *