Extend certificate functions to handle non-RSA keys + add support for ed25519 keys

[misberner]

This PR:

• adds support for Ed25519 keys in the genPrivateKey function. Note: This has been removed because it fails the go1.12 CI tests. While Ed25519 support is available in Go 1.12 via golang.org/x/crypto/ed25519, the x509.MarshalPKCS8PrivateKey does not support ed25519 keys and implementing it manually is reasonably complex. Before I do this, I would like to clarify whether the Go 1.12 tests are relevant at all given that the go.mod file actually specifies a minimum Go version of 1.13.
• adds support for non-RSA keys in the buildCustomCert function (in line with what Go supports, ECDSA and Ed25519 keys instead of just RSA keys)
• adds ...WithKey variants for the genCA, genSignedCert, genSelfSignedCert functions, that take as an additional parameter a PEM-encoded private key (any private key generated with genPrivateKey and an algorithm other than "dsa" works) to be used for the certificate
• adds a clarification to the documentation that the genCA, genSignedCert, and genSelfSignedCert functions use 2048-bit RSA keys.

The main motivation is to allow generating certificates with ECDSA or Ed25519 keys, which are much faster both in terms of key usage at runtime, as well as key generation time.

[technosophos]

This looks great

[misberner]

@technosophos thanks for the review! Do you have an opinion on the Go 1.12/ed25519 issue?

[mattfarina]

@misberner I think it makes sense to drop 1.12 support and add testing for 1.15. 1.12 has not been supported by Go in 10 months. I can do that in a follow-up.

[mattfarina]

The PR for that is at #272