add capability to create asset types via API

Like the title says, this allows API users to create, delete, and list AssetTypes.

I still need to add tests for this, but this will enable us to create new asset types to support different usage of collins assets (i.e. SERVICE or REPLICA_CONTROLLERs).

@Primer42 @maddalab 
These are all hardcoded in the evolutions, and are in some way or another used by collins internally.