Add provider for DNS-01 ACME challenge

[accuser]

Preliminary PR for addition of DNS-01 challenge provider.

Example configuration:

config:
  acme.agree_tos: true
  acme.domain: foo.example.net
  acme.email: [email protected]
  acme.challenge: DNS-01
  acme.provider: cloudflare
  acme.provider.environment: |-
    [email protected]
    CLOUDFLARE_API_KEY=XYZ

Closes #1364

[belthesar]

This looks pretty good! Thank you for doing this! I do have some feedback:

Per @stgraber's feedback here: #1364 (comment) , we should probably consider nestling config in keys specific to the challenge. My intention was to pattern this off of Traefik's config, an example found here: https://doc.traefik.io/traefik/https/acme/#configuration-examples
Given what you've done, I'd probably create a map something like:

acme:
  agree_tos: true
  challenge: "DNS-01"
  dnsChallenge:
    provider: "ProviderName"
    config: |-
      ENVVAR_ONE="value"
      ENVVAR_TWO="value"
    resolvers: "1.1.1.1,1.0.0.1"

This would set up a solid pattern if additional configuration options for challenges become available as well.

[accuser]

I think you're right, but I'd probably fall towards:

config:
  acme.challenge: DNS-01
  acme.dns.provider: cloudflare
  acme.dns.resolvers: "1.1.1.1,1.0.0.1"
  acme.dns.env: |-
      [email protected]
      CLOUDFLARE_API_KEY=XYZ

I'm favouring env for the environment variables, rather than config.

[stgraber]

Going to take a quick look at this one, for one thing, do a rebase and tweak the commit list :)

[stgraber]

Waiting for #1679 to merge, then this one can get rebased and will be good to merge.
I've just tested it here with a couple of DNS providers and it seems to be working as expected.