Support Curve25519 and Curve448 in the pki backend

[rgl]

Currently the pki backend only supports the NIST P-* curves but since those curves are not considered safe I would like to have Curve25519/Curve448 available in the pki backend.

Maybe it could be triggered with key_type set to ec-curve25519.

Please note that Curve448 is not yet available in go x/crypto or tls library.

[annerajb]

Considering ed448 is not available yet in go.

Would it be possible to split this issue into two separate ones ?

That way work on ed25519 could be done without waiting for ed448?

[cipherboy]

As contributed by @annerajb (thanks!) there is support for X25519 in the PKI secrets engine. Go Crypto team has declined to add support for X448; as such, we'll have to decline X448 support until such time as Go includes it. Using a third-party library is not a feasible approach for us.

Seeing such, I'm going ahead and closing. Feel free to re-open if Go's stance changes and support lands there. We'll happily add it then.