Remove Everit dependency

The immediate goal for this library is to allow us to avoid referring to Everit validator types directly in our internal projects at [Permutive](https://permutive.com/), and the 0.1 release accomplishes this goal.

There are two next steps: to identify what other functionality is necessary for other potential adopters, and to remove the Everit dependency, which will allow us to avoid the non-Maven Central resolver as well as the runtime overhead of converting to the `org.json` JSON representation.

No files, tests, or entry points are named. Start by locating the Everit validator and org.json conversion usage, then map the required validation functionality and Maven resolver configuration before deciding what removal entails; done means the library no longer depends on Everit or the non-Maven Central resolver while preserving its supported behavior.

Written by the indexing model from the issue text.