Add support for P-384 keys on the secure enclave

[maraino]

This commit enables the creation of NIST P-384 keys on the Secure Enaclave.

See last section of https://support.apple.com/guide/security/managed-device-attestation-security-sec8a37b4cb2/web

To create a hardware-bound key, the ACME configuration needs to use the ECSECPrimeRandom algorithm
with a key size of 256 or 384 bit. This specifies a key pair on the P-256 or P-384 curves as defined 
in NIST SP 800-186.