Apply provider-specific validation templates to work

[meilinger]

Might fragment some of the validation logic, but will expose issues up-front vs potential failures downstream

[garnertb]

So far, the validation seems provider-specific (ie https://github.com/FireCARES/harvester/blob/master/harvester/validators.py#L4 seems related to https://github.com/FireCARES/harvester/blob/master/harvester/providers/esri.py#L26). Maybe we just add a is_valid method to each provider? Am I missing something?

[meilinger]

This was more meant as a way to validate required fields in the work templates (based on specific providers)...might be overkill since we're falling-back to kwarg default values.