Refactor domain and resources

[meganwolf0]

The current validation structure has a somewhat confusing use of domain and resources, where they appear separate in the definition, but are coupled in practice - e.g.,

target:
  provider: opa
  domain: kubernetes
  payload:
    resources:
      - {list of resources}

This flows into the code organization - e.g., getting the resources is implemented redundantly inside the provider modules.

The structure of both the validation yaml and implementation in code should probably be refactored to reduce redundant usage and enhance clarity.