feat: generateKeyPairSync for x25519

[SamuelScheit]

• Added support for generateKeyPairSync x25519
• Added unit test
• Works on iOS
• Works on Android

Example code:

crypto.generateKeyPairSync('x25519', {
    publicKeyEncoding: { format: 'der', type: 'spki' },
    privateKeyEncoding: { format: 'der', type: 'pkcs8' }
});

[mrousavy]

Hey! Thank you for your PR! ❤️

This looks amazing, let us test a bit and see if everything works as expected!

[EternalDeiwos]

Any chance this could also include ed25519?

[boorad]

@SamuelScheit hiya, any chance you could rebase this onto main? I'm actively working issues/PRs now, so would love to get this in! 💪

FYI, I'm using some of your ideas in #304 (i.e. KeyVariant) so there may be conflicts.