To prepare for the upcoming certmagic update in Casket, this PR updates all of the DNS providers to use libdns instead of lego, where possible. While it is technically possible to write a wrapper for lego, I thought it would be better to switch to what certmagic actually supports, and reduce our maintenance footprint too.

Over half of the DNS providers were removed, and some configuration options have changed. This is definitely a breaking change. I'm not sure how many people outside of tmpim use Casket, so we will need to clearly communicate the breaking changes, and invite issues/pull requests for providers that libdns doesn't support. Worst case, we can write a lego wrapper. I have documented the changes to the DNS providers and their configuration here, which can be included in the release notes when the time comes.

This change also supports block configuration for all of the DNS providers, which allows us to account for the only change we made in DNS providers - supporting more than one configuration/key per each type of provider. Environment variable expansion should still work in these block configurations. An example configuration for the cloudflare provider would look like this:

example.com {
  tls dns cloudflare {
    token ...
  }
}

This PR depends on changes in Casket (tmpim/casket#29), but this one will need to be merged first. I will also update the casket-docs for every provider.