Make `FirebaseSubscription` and `FirebaseQueryResponse` declarations coherent

[yevhenii-nadtochii]

FirebaseSubscription and FirebaseQueryResponse are both used as retuning types for the pre-defined servlets. They say where the actual response data is expected to be (Firebase node). And there is a disharmony in how they declare path to a database node:

FirebaseSubscription uses a plain string:

    // A database path to the requested data.
    string path = 1 [(required) = true];

FirebaseQueryResponse uses more specific NodePath:

    // Path in Firebase RDB where the records matching the subscription are stored.
    NodePath node_path = 2 [(required) = true];

Can we have these declarations coherent, so they use the same types for similar fields? BTW, FirebaseClient also uses more specific NodePath instead of String in its API.