python: interface file for envoy_engine.so (#1244)
python: interface file for envoy_engine.so (#1244)

Description: I believe mypy and other static analysis tools can't introspect a `.so` file for a Python library's types. This PR introduces a mypy stub file (https://mypy.readthedocs.io/en/stable/stubs.html) to provide type information for both raw usage, and for typechecking the gevent & asyncio wrappers.
Risk Level: Low

Signed-off-by: Cerek Hillen <9f0a05fabc80966375f922248b0ac191ad4b7262@lyft.com>
envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile,envoyproxy/envoy-mobile