Implement logic for loading user generators into fastapi-mvc CLI

[rszamszur]

• I have searched the issues of this repo and believe that this is not a duplicate.

Feature Request

Once a user creates a generator (#68), it needs somehow to be loaded into available fastapi-mvc generators.

[rszamszur]

This feature is made of 2 parts:

1. A method to programmatically load generator modules at runtime.
2. A way to programmatically create click.Commands based on available generators (and their options), which will be imported via loader mentioned above.

Resources:

• https://docs.python.org/3/library/importlib.html#importing-programmatically
• https://docs.python.org/pl/3.7/library/pkgutil.html
• https://click.palletsprojects.com/en/8.0.x/commands/#custom-multi-commands
• https://click.palletsprojects.com/en/8.1.x/api/?highlight=click%20multicommand#click.MultiCommand
• https://click.palletsprojects.com/en/8.0.x/api/#click.CommandCollection