Get Python code closer to PEP 8 standard

[Irillit]

I focused here on:

• Changed camelCase to snake_case in variables, fields, methods and functions;
• Renamed literals that shadowed built-in functions
• Added docstrings
• Renamed types.py to schemas.py, as types.py shadows a built-in name. I called it schemas, as it contains Pydantic models (we can call them schemas), and models might be confused in this context with ML models.
• Split init.py in veeneer folder
• Moved some repeated strings to class constants;
• Some other changes (context managers instead of open/close, list comprehension instead of map, Optional type instead of type | None)

Plans for the future:

• Split vertex.ai plugin from init.py to normal modules;
• Make the structure of py folder more Pythonic
• Separate the constructor of Actions into other methods