[wip][demo] uses crypto traits and allows fxa_client to compile to wasm

[tarikeshaq]

What is this

Historically we've only used a single backend for crypto, NSS. Overtime, nss found its way to so many of our crates. That's ok for our existing use cases because we are building binareis for Firefox and we should only be using NSS in firefox.

However, if we ever want to make some of our crates available for the web as wasm, our dependency on NSS becomes a problem (I don't think there is any desire to make NSS compile to wasm)

This is a draft PR that acts as a starting point to create pluggable crypto into our components.

Crypto traits

At its core, I'm suggesting defining traits that represent all the crypto operations we are currently using through NSS, and then having our individual crates depend on those traits. We have two types of crates, supporting non-exposed crates (eg: sync15), and exposed crates (sync_manager, fxa_client, places, etc). I'm suggesting the following:

• For supporting crates, they should not have a direct dependency on any crypto library, and instead depend on crypto_traits and be generic over any implementation of the crypto operations the crate needs
• For exposed crates, they should make nss (and rc_crypto) an optional dependency that is enabled by default but enable easily switching the crypto implementation by making private APIs generic over any crypto backend, but have public APIs use whichever backend is enabled

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.08%. Comparing base (01a52c7) to head (e19a3fc).
Report is 6 commits behind head on main.

Additional details and impacted files

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  Coverage   84.08%   84.08%           
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  Files         117      117           
  Lines       15629    15629           
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  Hits        13141    13141           
  Misses       2488     2488           

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