Promote experimental FIL and maintain current FIL as LegacyFIL

[csadorf]

Promote the experimental FIL (Forest Inference Library) implementation to replace the current FIL implementation while maintaining the current implementation as LegacyFIL. The RandomForest estimator will continue using LegacyFIL while we sort out some compatibility issues.

This change aims to:

1. Make the experimental FIL implementation the default for new users
2. Preserve existing RandomForest functionality by continuing to use LegacyFIL

Note: This change is focused on the Python API layer and does not affect the underlying C++ implementation.

Checklist:

• Demote current FIL into legacy namespace
• Update RandomForest estimator to use LegacyFIL internally
• Add deprecation warning to LegacyFIL class
• Update documentation to reflect new FIL as default implementation
• Review any current deprecation warnings in FIL
• Add performance benchmarks comparing both implementations