Feature/validator options metadata

This PR adds the following:

- A list of validator metadata. This list is keyed by the validator plugin names, with a list of option name/type pairs. This will allow us to provide selections for options based on the validator, as well as appropriate value inputs for each option. Validator metadata is provided via the ValidatorMetadataModel class.
- The ValidatorsModel has been updated to consume a ValidatorMetadataModel and to merge its list of plugins with that metadata when returning the list.
- Client-side, this adds a factory for the Validator service endpoint. The REST services controller now consumes this endpoint and assigns the data to the scope.