observability: support log levels for native/platform logging

Right now we don't have a way for platform (Swift/Kotlin) or native (ObjC/Java) to emit logs through Envoy in a way that respects their log levels specified in configurations. Thus far, we've added occasional `NSLog` statements, but it'd be great if we could send these logs through the same sink that other Envoy logs go through.

The main point of discussion with this change is that it would require native code to depend on upstream Envoy's logger/logging interfaces. This might be fine, but is something that should be decided.

More discussions:
- https://github.com/lyft/envoy-mobile/pull/709#discussion_r386692846
- https://github.com/lyft/envoy-mobile/pull/701#pullrequestreview-362978207