[ML] Convert encodeIds flag in TreeEncoder to configuration property

[AliRana30]

Summary of Changes

• Configurable ID Encoding: Replaced the hardcoded final boolean encodeIds = false; in TreeEncoder with wayang.ml.encoding.encode-ids (defaulting to false, with shorthand wayang.ml.encode-ids supported).
• TreeEncoder Enhancements: Added TreeEncoder(mappings, configuration) constructor, getter/setter for configuration, and overloaded encode methods that resolve encodeIds from OptimizationContext or local configuration.
• Default Properties: Loaded wayang-ml-defaults.properties in MachineLearning.plugin().setProperties(Configuration) and declared wayang.ml.encoding.encode-ids = false.
• Unit Tests: Added test cases in TreeEncoderTest verifying default behavior, enabled ID encoding via configuration, and plugin properties loading.

Closes #776.

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