df: types: Seperate validation from Defintions

[johnandersen777]

Definitions are supposed to be independent serializable structures. However, we've added the validation parameter which can be a function or an instance name.

The issues with this are

1. validation functions can't be serialized, since they're functions
2. operation instance names are tied to dataflows

As a side note: remove_unvalidated_inputs and remove_input should really be async functions. Also, they should be required method defined within the abstract base class.