Summary:
Adds hybrid JNI wrapper class that can hold a native delta client.
This class allows Java code in the `devsupport` package to hold onto a native delta client across reloads.
It also takes care of passing `okhttp` response objects to native code, where we build a `std::string` to hold JSON messages coming from Metro, and parse them with `folly::parseJson`.
In a follow-up, we will switch to a streaming-friendly binary format that will make allocating memory for the whole message unnecessary.
Reviewed By: fromcelticpark
Differential Revision: D7845138
fbshipit-source-id: 7a29b30645331addf843097dd0d05c03b3143991
