Add `ES256K` algorithm

[daniel-mader]

Are you planning to add more ECDSA algorithms such as ES256K (curve secp256k1)? I looked into ring which currently only seems to support ES256 and ES384. Seems like RustCrypto/elliptic-curves would support it, but I reckon that you would not consider adding the additional k256 crate at this point.

[Keats]

Not for now, there are a few PRs to change the backend already:

1. Add feature based on aws-lc-rs cryptographic library instead of ring #377
2. remove ring #318

[ThisIsMissEm]

@Keats is there any updates on this? looks like both PRs mentioned above are either merged or closed?

I'd love to get this implemented as it makes using the jwt-cli to debug the Bluesky / AT Protocol access tokens possible. (they use ES256K)

[ThisIsMissEm]

Actually, I think I might even be able to do a PR to implement this, using the RustCrypto implementation of k265

The majority of the code looks like it'd actually be in the tests.

[Keats]

It should be fine to add yes

[ThisIsMissEm]

Okay, well, I tried but I'm probably too inexperienced to get this right: https://github.com/Keats/jsonwebtoken/compare/master...ThisIsMissEm:jsonwebtoken-rs:feat/implement-secp256k1-ES256K?expand=1

There's currently an error in the test to do with the private key being in an invalid format, this was a key that was exported from Node.js via the @atproto/crypto package, and the public key is the public key exported as jwk.

Maybe it's a starting point for someone else though?