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