[hydra] Replace Marshmallow with Pydantic

[bolinocroustibat]

Replace Marshmallow with Pydantic

Why this change:
• Broad adoption (notably in FastAPI), excellent documentation, and active maintenance.
• Better performance in many use cases (see discussion: HN – Why Pydantic is replacing Marshmallow).
• Pydantic relies on native Python type hints, making code more concise and readable, and compatible with tpe checkers/IDE
• Cleaner object-oriented approach, with models closer to enriched dataclasses.

PR: #149