[Serve] Refactor to Fix Type Checking Errors

[andylizf]

This PR refactors multiple files within the Serve module to address type-checking errors and improve type annotations. Key changes include:

• Adjustments to method signatures and variable types in modules like autoscalers.py and replica_managers.py for better type safety.
• Modifications to ensure consistent use of aiohttp.ClientTimeout in load_balancer.py to prevent timeouts.
• Refactoring of methods in serve_state.py to make paths considered as str rather than str | Path using helper functions.
• Cleaning up and streamlining processes in service.py for managing service states and cleanup tasks.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[cblmemo]

@andylizf Thanks for the refactor! This mostly looks good to me. Left some nits

[cblmemo]

Thanks for the quick fix @andylizf ! Left one last nit and after that it should be ready to go 🙌