Perform keyauth hashing in AddDNSOneChallenge, add ACME prefix

[cpu]

Presently the AddDNSOneChallenge function is quite simple and adds the provided content under the provided host:

challtestsrv/dnsone.go

Lines 9 to 15 in 285efd6

	// AddDNSOneChallenge adds a TXT record for the given host with the given
	// content.
	func (s *ChallSrv) AddDNSOneChallenge(host, content string) {
	s.challMu.Lock()
	defer s.challMu.Unlock()
	s.dnsOne[host] = append(s.dnsOne[host], content)
	}

That means callers have to add the _acme-challenge. prefix for RFC 8555 DNS-01 challenges to the host themselves. They're also responsible for hashing the key authorization for the content argument.

Since this is a function specifically for DNS-01 it should do more of this work automatically. This is an API breaking change and will need to bump the release accordingly.

[cpu]

It would probably be worthwhile to take this as a chance to holistically review the whole API and make similar improvements for TLS-ALPN-01 and HTTP-01 as appropriate. I think there could also be some consistency changes made in naming.