Implement Serializable to support serialization via ObjectOutputStream

[ylwu-amzn]

Currently MLCommons is using ObjectOutputStream to serialize object (code link). But ThresholdedRandomCutForestState doesn't implement Serializable, so it's impossible to serialize via ObjectOutputStream.

AD is using protostuff to serialize. protostuff has one bug reported in AD opensearch-project/anomaly-detection#263, we fixed by adding local jar dependency as protostuff team has no clear plan to release new version.

If we use same way in MLCommons we need to add one more dependency and we need to maintain two serialization ways. So prefer to implement Serializable to make serialization easier.