certgen: Support Ed25519 keys

[jrick]

For decred/dcrwallet#1475 (and presumably dcrd too) we wish to support Ed25519 keys in TLS certificates. Currently, certgen exposes the function:

func NewTLSCertPair(curve elliptic.Curve, organization string, validUntil time.Time, extraHosts []string) (cert, key []byte, err error) {

which is not suitable for other signature schemes other than ECDSA.

Either a breaking change is necessary (to pass a private key as crypto.Signer and switch on its type) or a new API is needed in order to enable support for Ed25519 keys.

crypto/tls enables Ed25519 support in Go 1.13 so conditional compilation is necessary to not break the 1.12 build.