Streamline event tracker registration

There's some room for improvements in the event tracking registration and consumption part of the code.

Here's the improvements to make:

1) Add a `isRegistered(std::string name)` method to [external.h](https://github.com/envoyproxy/envoy-mobile/blob/main/library/common/api/external.h)
2) Ping one of the maintainers to merge https://github.com/envoyproxy/envoy-mobile/pull/1673
3) Map the change in step 2 for iOS in [EnvoyEngineImpl.m](https://github.com/envoyproxy/envoy-mobile/blob/main/library/objective-c/EnvoyEngineImpl.m#L453-L461) - do not call `register_platform_api` when `ios_track_event` is null