Opensc curve25519 compatibility

[sosthene-nitrokey]

pkcs11-tool --login --login-type so --keypairgen  --key-type EC:prime256v1

Works, but

pkcs11-tool --login --login-type so --keypairgen  --key-type EC:edwards25519

doesn't with error: Generate EC key mechanism 1055 not supported'

and neither does

pkcs11-tool --login --login-type so --keypairgen  --key-type EC:curve25519

with error: error: Generate EC key mechanism 1056 not supported

[sosthene-nitrokey]

Relevant previous issue with RSA: #96

[sosthene-nitrokey]

Note: openpgp-tool does not support generating any type of EC keys, so this is why this works with pkcs11-tool