Mitmproxy-rs

- Make server shutdown more robust.

- UDP connections are now modeled as streams.

- Renamed `OsProxy` to `LocalRedirector`.

- Various improvements to the `mitmproxy-macos` source distribution.

- `mitmproxy-macos` is now also distributed as a source distribution on PyPI.
The source distribution contains some pre-compiled binaries built by our CI.
This is necessary because the network system extension needs to be signed &
notarized with a valid Apple Developer ID before it can be installed.

- Raise an ImportError if platform-specific dependencies are missing.