Introduce fully typed client

[vdusek]

Currently, the apify-client-python relies mostly on untyped dict and poor typing in general.

Let's introduce full type coverage across the client by defining Pydantic models for API responses and adding proper type annotations for all functions, methods, and their arguments.

Where possible, these models and types should be generated directly from the OpenAPI specification to ensure consistency with the API.

OpenAPI specs - https://docs.apify.com/api/openapi.json

Some existing solutions to explore & test:

• https://github.com/openapi-generators/openapi-python-client
• https://github.com/OpenAPITools/openapi-generator
• https://github.com/koxudaxi/datamodel-code-generator
• https://github.com/MarcoMuellner/openapi-python-generator

[shubhamos-ai]

This looks like a valuable enhancement! The proposed approach seems solid. I'm available to help with coding or testing if needed.