platform: callback threading specification

The bridge code currently leaves it up to the user to specify in which threading context the response callbacks are handled. If the user decides to provide a naive specification, Envoy Mobile could execute responses on Envoy's main thread. For example, in Android by letting the Executor run on the context the callback is first received (which is Envoy's main thread).

One option is for us to either provide a default threading specification and allow users to rely on the default we provide or use the threading specification of their choice (and allow them to also use a naive specification)