Refactor: switch to ring library

[flavio]

Description

A long time ago we moved away from the ring crate to a constellation of pure-rust cryptographic libraries. We did the switch because the ring library did not build for certain architectures (like s390x and webassembly).

This limitation has been addressed by latest versions of the library. Moreover, we have recently reintroduced the ring dependency to implement TUF trustroots.

Moving back to ring would reduce the list of dependencies we have, making the codebase easier to understand and to maintain.

I think these are the dependencies we should be able to remove:

• ecdsa
• ed25519
• ed25519-dalek
• elliptic-curve
• p256
• p384
• pkcs1
• pkcs8
• rsa

[jleightcap]

@flavio @lukehinds ToB is willing to pick this up!

[flavio]

@jleightcap: awesome, who should assign this issue to?

[flavio]

Update: rustls is now supporting two different crypto backends: ring and aws-lc-rs. The latter one is a drop-in replacement of ring that provides FIPS support.

The tough crate is also considering to perform the same change.

I still think we should drop the pure-rust libraries and support either ring or aws-lc-rs

[viccuad]

I still think we should drop the pure-rust libraries and support either ring or aws-lc-rs

It seems that aws-lc-rs is finicky for Windows builds. Since they are drop-in replacements, I think it would be a good idea to provide a feature for selecting between them.