Implement categories api

[ThomasRedpanda]

Documentation: https://apidocs.basekit.com/api-reference/categories/

Module:

• Add a CategoriesApi in the Api namespace.
• Add the module to the BaseKit class.

Endpoints:

• get categories

When implementing the endpoint, make sure to include links to the documentation using the @see annotation in the docblock.

Testing:

• Add tests for all the added domain objects
• add tests for the added endpoint