optional: Add support for ed448

ed448 is a signing algorithm being standardized by CFRG which operates similarly to ed25519 but has a higher security level. This algorithm is part of TLS 1.3.

This is a nice to have feature, rather than requirement.

https://tools.ietf.org/html/rfc8032

Parts:

• Add in nettle
• Add as a general signature algorithm in gnutls (X.509/PKCS#7)
• Add in TLS
• Test suite for nettle
• Test suite for X.509/PKCS#7 verification / generation
• Test suite for TLS use