feat: Add Meta.literal_keys option

[rodrigobnogueira]

Summary

Add literal_keys Meta option to allow field names containing double underscores without triggering deep context parsing.

Fixes

• Closes Is there a way to disable "deep context" for a factory? #1118

Changes

• Added literal_keys option to FactoryOptions
• Updated DeclarationSet to respect literal keys
• Improved error messages: When an InvalidDeclarationError is raised due to deep context parsing, the error now includes a helpful hint suggesting Meta.literal_keys=['field__name']
• Added tests and documentation

Example

class ServiceFactory(factory.Factory):
    class Meta:
        model = Service
        literal_keys = ['service__name']
    
    service__name = "foo"

Verification

• All 442 tests pass
• Linting clean (make lint)
• New tests added in tests/test_base.py:
  • test_literal_keys: verifies keys are preserved without splitting
  • test_literal_keys_inheritance: verifies option inheritance
  • test_literal_keys_missing_raises_error: verifies helpful error hint