Factor out JSON to repeated protobuf conversion

In general, we have the collection of protobuf messages as another protobuf message, which makes JSON -> protobuf conversion straightforward. This is not always the case, for example, Resources class is not a protobuf, though protobuf-convertible.

To facilitate conversions like JSON -> Resources and avoid writing code for each particular case, we propose to introduce JSON::Array -> repeated protobuf conversion. With this in place, JSON::Array -> Resources boils down to JSON::Array -> repeated Resource -> (extra c-tor call) -> Resources.