DNSSEC signer (#866)

DNSSEC signer (#866)

* first cut at key_to_dnskey

* update docs

* typo

* use real test vectors for DNSKEY

* comment

* split

* add test for large exponent size

* rename to make_dnskey

* no default algorithm

* rename and add comment

* split out function to create rrsig signature data

* docs

* add type for public key

* more typing

* make RSA exponent key test easier to read

* work in progress for dns.dnssec.sign

* better docs

* docs

* simplify

* add test with RSASHA1

* initial support for DSA

* update docs

* clean up DSA, t still not clear

* allow inception/expiration to be specified as datetime, string, float or in

* allow rrset to be specified as a tuple

* calculate dsa_t

* reformat

* more rrset tuple fixes

* support DSA

* improve exception handling

* fix return type error

* fix typing issue to silence mypy

* make test case more verbose

* ensure UTC and use sigtime_to_posixtime to convert text to timestamp