add ClientTransportPlugin Interface

[cyBerta]

I'm currently integrating IPtProxy into a fork of tor-android, so that I can easily run tor over snowflake using tor-android.
I wonder if it is of interest for this project to contribute back these little changes.

Instead of adding the IPtProxy to tor-android, I chose to add an interface containing start(), stop() and getTorrc(). Library users can implement this interface to inject a ClientTransportPlugin, to start the ClientTransportPlugin before tor starts, to stop it accordingly and to define the necessary torrc string.

All that is still WIP, but you can have a look at https://0xacab.org/leap/android_libs/tor-android/-/blob/main/tor-android-binary/src/main/java/org/torproject/jni/TorService.java and https://0xacab.org/leap/android_libs/tor-android/-/blob/main/tor-android-binary/src/main/java/org/torproject/jni/ClientTransportPluginInterface.java

Feel free to close this ticket if you think it's not worth a PR.

[n8fr8]

We are working on our own pluggable mechanism to do this. You would install a separate Snowflake-PT APK, that the TorServices/tor-android library would discover, launch and link into. Our goal ultimately is to allow for truly pluggable PT's, and not compiling in a specific dependency.

That said, I'll take a look at your approach and see how it might integrate with what we have planned.

[cyBerta]

Thanks for your fast response and for taking time to look at the proposed change.

Our goal ultimately is to allow for truly pluggable PT's, and not compiling in a specific dependency

That sounds reasonable and and on the library level this goal is met with the proposal.