Adding support for ECDSA NIST curve SSH keys

[fakegermano]

As they are becoming more common in the field, This PR adds support for the use of that type of key for encryption/decryption.

This PR also makes use of the new go 1.21 ecdh package that natively implements the key exchange for elliptic curves (supports all X25519 and P256, P384 and P521).

I can also add similar PRs for the rust and typescript implementations when I have time, but let me know if I have missed something or messed something up, or If I need to add more tests :)