Add functionality that enables configurable secrets sources, with a
hard-coded `env` that provides the existing environmental variable
support.

Both point to the same thing via properties

- All of the `if data isn't a list turn it into one in the value type
  validates are no longer needed, they'll always be passed a list now`
- Special case to handle PTR/target values since it previously was
  single value and is now values

See README for more information

Create two new filters, ValueAllowlistFilter and ValueRejectlistFilter
that allow checing the value(s) of records to include or exclude,
similar to the name filters that alread exist

- This makes it possible to use dynamic zone config with AutoArpa
- Document what gotchas that entails
- Add tests both with another source (for generating zones to populate) and without (no zones to populate)