[FEATURE] Dedicated crates for wadm types, wadm API

[protochron]

After chatting with @brooksmtownsend, we realized it might be useful to have dedicated crates for the Wadm API and for the various event and model types that we use. While they're all public types and can be consumed by other software, having them exposed as separate crates would allow downstream consumers to not have to depend on the entire wadm codebase in order to make an API call or parse a manifest. It could also be nice for introducing new features or changes using cargo feature flags and generally just decouple the data that wadm exposes from the underlying scheduler codebase