DNSSEC signer

[jschlyter]

RRset signer

This PR implements a RRset signer via the following new functions:

• make_dnskey() - convert a cryptography public key to DNSKEY Rdata (also available as Create DNSKEY from public key #864)
• sign() - Sign RRset using a cryptography private key

Implementations Notes

The logic for calculating signature data (_make_rrsig_signature_data()) is shared between _validate_rrsig() and _sign().

Signer timestamps can be specified as datetime, string in YYYYmmddHHMMSS format or as an int/float.

The signer implements all RSA, DSA, ECDSA and EDDSA algorithms, but does not implement GOST.

Testing

• Key test vectors generated using OpenSSL has been added to tests/keys.py. Matching DNSKEY RRs was generated using Knot DNS.
• Signature testing implemented for all algorithms (generate key, sign, validate)

Closes #864
Discussed in #862