ios/android: add support for native L7 filters

In addition to supporting the core Envoy C++ L7 filter chain (https://github.com/lyft/envoy-mobile/issues/543), we plan to add new interfaces in Swift/Kotlin that allow consumers to write and register L7 filters from the platform APIs.

This will decrease the barrier to entry for writing Envoy Mobile filters, and allow consumers to utilize platform-specific APIs with their filter implementations.