engine: explore the possibility to implementing pieces of the engine in rust

[hellais]

This is related to the engine simplification work.

The reasons for doing this are:

• The story of integrating rust with mobile apps is much nicer (see: https://github.com/mozilla/rust-android-gradle)
• The shipped library is going to be significantly smaller
• We can do parroting in a way that is cleaner and more robust than with utls (see: https://crates.io/crates/rquest)
• Work related to probe-id could benefit from using the already existing cryptographic code that's rust based (see: https://github.com/nategraf/rkvc/ & https://gitlab.torproject.org/onyinyang/lox-zkp)
• Tor integration has been a long source of pain and the rust based arti rewrite would help us with that

This issue is about doing exploratory research to understand the limits of this.

The order or priorities to implement this would be to first focus on the probe-services components which we need for the anonymous credentials project and then eventually move over to experiments (which are going to take a very long time to fully rewrite in rust).

In the interim the tentative plan is to ship both a golang and rust library to the apps which we load dynamically and call either rust or go code depending on what we need.

This is related to: #2826

[hellais]

Work on this has started as part of https://github.com/ooni/ooniprobe-rs/.

So far the pieces that have been explored are:

• How to parrot TLS
• How to expose a nice JNI to kotlin
• How to do request tracing to measure timing information (this is relevant for experiments)
• How to mimic the ooni data formats

Based on these initial findings the direction seems quite promising. The most delicate layer seems to be that of integrating it into the kotlin codebase, though it's nice that common rust idioms of method chaining translate naturally over to kotlin. The main pieces that are a bit tricky are those of wrapping and passing around raw pointers in the JNI bindings making sure this stuff is memory and thread safe.

Next things to look at in relation to jni are:

• https://github.com/jrobsonchase/thrush/blob/master/src/macros.rs
• https://github.com/Dushistov/flapigen-rs