PR-6377 Add base ummg generation classes

[reweeden]

Adds a stripped down version of our UmmgBase class to mandible. The idea is that the UmmgBase class has a get_* method for every possible UMM-G element. Elements that are required either get a default implementation if one makes sense or are marked as abstract methods. Workflows can define their own WorkflowUmmgBase subclasses to add additional workflow specific functionality to the base class.

Some optional elements that have nested fields also get a mixin class that can be used to pull in boilerplate get_* methods for sub elements. Some even provide a default implementation if that implementation was common enough in our workflows.

The idea for now is just to create a pretty bare bones starting point. There may be more functionality that is common enough to pull into mandible later. There is already a lot of code here because of the TypedDict definitions which give useful type hints and the large number of boilerplate get_* functions.

[mattp0]

I like the approach!