[engine-simplification] Support tunnels in rust http client

[DecFox]

This issue is about supporting tunnels in the rust http client we aim to expose to the apps. This should allow us to expose a complete http client via rust directly and remove the FFI layer we use to communicate between go and rust. The apps should then be able to call the rust http client directly for all backend communication.

The deliverables for this issue include:

• Support psiphon, tor and other tunnels currently in ooniprobe in the rust client in a stateless manner
• Expose the rust api client interface to be used by the multiplatform app