[FEATURE] Migrate core parts of the library to use Pydantic

[20001LastOrder]

PROBLEM
Currently, only a subset of modules in Sherpa uses Pydantic for data validation. We should migrate the core part of the library to use Pydantic to enable data validation and make the code more robust. This includes:

• agents
• actions
• memory and belief
• validator

This will give us a unified way of instantiating things and doing type checking.