This PR changes the import_app function to accommodate the following cases:

openbb-api --app my_module.main:app
openbb-api --app main.py:app
openbb-api --app my_folder/main.py:app
openbb-api --app main.py --name another_name
openbb-api --app my_module.main:create_app
openbb-api --app main:create_app --factory

and others.

So now the launcher can handle python modules, sub-modules, relative paths, absolute paths.
The factory launcher now uses similar logic to what uvicorn uses to detect the factory allows us to stop relying on explicit type annotation of the factory function.

Additinonally some tests for import_app were added, although 2/3 of them actually work