Support generic models

[rafalkrupinski]

Generic models can be either custom fields object, as in

class Model[T]:
  field1: str
  field2: int
  metafields: T

or

class Model[T]:
  id: uuid.UUID
  created_time: dt.datetime
  payload: T

or the entire model can be custom, and only the operations are the same, as in database-driven services.

Essentially, from the service-provider perspective, such service is only partially defined, and the full definition varies from user account to user account.