Allow downstream users to be provider agnostic

[jcgruenhage]

Right now, a developer using this library has to have some code selecting which provider to use and has to feed the parameters into the API exposed by this library themselves.

Ideally, a developer using this library could call DnsUpdater::from_env(), or something similar with a type implementing serde::Deserialize, which would give back a DNS updater based on the deserialized config or config loaded from the environment variables. This way a developer wouldn't have to care about which underlying implementation is being used at all.