feat: add customFields property to Experiment

[vazarkevych]

Summary

Adds support for Custom Fields in Experiments, allowing developers to access
custom metadata defined in the GrowthBook UI through the Python SDK.

What Changed

• Added customFields property to the Experiment class (defaults to {})
• Parsed customFields from the API response during Experiment construction
• Serialized customFields in to_dict() when non-empty
• Added unit tests covering all scenarios

Why

GrowthBook UI allows defining Custom Fields for experiments (e.g. team
ownership, ticket links, metric type). The API returns these under a flat
customFields object, but the Python SDK previously dropped the field during
parsing, making it inaccessible to developers even when the API returned it.

Test plan

• test_custom_fields_parsed_from_api_dict — parses string and numeric values from the API dict
• test_custom_fields_default_when_absent — defaults to {} and is omitted from to_dict() when empty
• test_custom_fields_public_init — works via the public constructor

All 545 tests in tests/test_growthbook.py pass.