Store private keys in the DB as PEM blocks rather than pieces of the BIND Priv Key Format

[johanix]

In tdns/sig0_utils.go:GenerateKeyPair() a keypair is generated and subsequently stored in the KeyStore DB. However, only ED25519 keys can be stored at the moment, because of unfortunate assumptions about the key being stored in a BIND9 Private Key Format v1.3.

• This code should be updated to instead convert the private key into a PEM block and store the PEM block in the DB.
• A new function must be created that extracts the PEM block from the DB and from that creates a crypto.PrivateKey that may be used in the rest of the code.
• Then all consumers (i.e. users of the private key) need to be located and updated to use the new access function instead of the immediate ED25519 private key.