Add Online Isolation Forest Support

Implement a fully online version of Isolation Forest per the paper “Online Isolation Forest” (https://arxiv.org/abs/2505.09593). The library can learn and forget on the fly over a sliding window in Spark / Scala pipelines.

Key points
• New OnlineIsolationForest estimator + mutable model that expose partialFit / sliding‑window updates.
• JSON model schema v2 and optional snapshot‑to‑ONNX exporter (batch export path remains unchanged).
• Structured‑Streaming helpers for real‑time scoring; legacy batch workflows remain unaffected.