Crypto library support for compressed public key representations of supported elliptic curves (eg secp256k1, secp256r1...)

[daidoji]

Is your feature request related to a problem? Please describe.
It is not directly related to any particular problem, its just some functionality that would be useful in interacting with other libraries that provide the compressed forms of ecc public keys rather than the uncompressed format that the crypto library currently provides.

Describe the solution you'd like
It would be nice if there were a flag or method that could take public keys of elliptic curves (ie secp256k1, secp256r1,...) and provide the compressed representation of that public key rather than the uncompressed version which is currently supported by generate_key. crypto is compatible with https://datatracker.ietf.org/doc/html/rfc3279.html as the uncompressed form is a MAY requirement, but many libraries online now use this representation so it would be nice to have in the standard library.

What happens now:

iex(1)> {public_key, secret_key} = :crypto.generate_key(:ecdh, :secp256r1)
{<<4, 247, 16, 231, 77, 232, 141, 236, 250, 218, 164, 80, 186, 117, 170, 186,
   220, 12, 242, 175, 151, 151, 201, 163, 56, 254, 57, 224, 210, 80, 243, 120,
   138, 7, 217, 80, 222, 72, 45, 167, 220, 218, 221, 13, 212, 133, 30, 129, 79,
   ...>>,
 <<232, 164, 104, 253, 217, 81, 96, 138, 146, 100, 233, 120, 67, 34, 104, 3,
   170, 232, 161, 232, 30, 159, 180, 164, 227, 245, 163, 83, 132, 237, 127,
   133>>}
iex(2)> {byte_size(public_key), byte_size(secret_key)}
{65, 32}

With the compressed format of the public key to the RFC, it would be {33, 32} for that last line.

Describe alternatives you've considered
We have looked for alternative library support in hex but have not yet found it and are currently implementing the compression/uncompression of these public key representations in our application. We may also look to NIF bindings to Rust libraries which do provide this functionality.