Generally, I think helper functions like this create_lookup_field method ought to be available as extensions and not baked into the official SDK.

Dataverse message operations are defined as names, and there is no CreateLookupAttribute message. You create a lookup while creating the relationship with the CreateOneToManyRequest Class

This specific named action isn't present in OData, but it could be. Instead, we implemented POST to /RelationshipDefinitions

I think we ought to do the work to add the underlying Metadata Entity Types so that Jeff (or anyone) could use them to compose these types of operations on top of a message named create_one_to_many_relationship rather than starting with create_lookup_field.