refactor: workflow type/version validation

[uniqueg]

Validating incoming workflow run requests against the currently supported worklow types/versions invoked a call to the GET /service-info endpoint to get the latest info on supported types/versions. However, the way this is currently implemented bears the risk of circular imports.

In particular, it would be nice to get rid of the dedicated pro_wes.ga4gh.wes.states module in favor of the State enum in pro_wes.ga4gh.wes.models, but this currently causes a circular import. Attempt to refactor such that no circular import arises.