Feat (common/quantizer): Enable passing custom quantizers

[pablomlago]

Reason for this PR

Currently, every time we need to support a new quantization type, we need to modify our entry-point in several ways (e.g., new args, new option in the dict), and this process does not scale up with more advanced quantization schemes.

Changes made in this PR

• Added a generic Registry helper to python_utils.py with a decorator-based API.
• Created base class for custom quantizers (BaseQuantizer).
• Added new custom-quantizer argument to the LLM entrypoint.
• Added example demonstrating custom quantizer registration using the new registry.

Testing Summary

• Added new test in test_llm_cases.py.

Risk Highlight

• This PR includes code from another work (please detail).
• This PR contains API-breaking changes.
• This PR depends on work in another PR (please provide links/details).
• This PR introduces new dependencies (please detail).
• There are coverage gaps not covered by tests.
• Documentation updates required in subsequent PR.