[comm-opaque] Create opaque 2.0 facade for usage in Comm

Summary:
This exposes our usage of opaque 2.0 in a way that
many of the details are obscured from the calling code.

This should eventually allow for significant reduction
in code re-usage in current code base, as well as
open the door for exporting the facade as a wasm api.

https://linear.app/comm/issue/ENG-3277
https://linear.app/comm/issue/ENG-3148

Related: https://linear.app/comm/issue/ENG-3265

Depends on D7020

Test Plan:
Full end-to-end testing would need to be done in another diff.

However, there is a unit test in lib.rs which goes through a register + login workflow in rust.

cd shared/comm-opaque
cargo test

Reviewers: varun, bartek, O1 Nix

Reviewed By: varun, O1 Nix

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7022