restored internal_config feature as part of "unstable" (#1660)

zenoh/Cargo.toml

@@ -64,7 +64,7 @@ transport_udp = ["zenoh-transport/transport_udp"]
 transport_unixsock-stream = ["zenoh-transport/transport_unixsock-stream"]
 transport_ws = ["zenoh-transport/transport_ws"]
 transport_vsock = ["zenoh-transport/transport_vsock"]
-unstable = ["zenoh-keyexpr/unstable", "zenoh-config/unstable"]
+unstable = ["internal_config", "zenoh-keyexpr/unstable", "zenoh-config/unstable"]
 internal_config = []
 
 [dependencies]