Feature Quantization [3/5]: Preprocessor (write-side) Updates

[jchmura-sc]

Purpose
The purpose of this PR is to make the (write-side) preprocessor changes for feature quantization:

• FeatureQuantizationSpec the API endpoint that the user can specific inside NodeDataPreprocessingSpec
• beam transform for computing quantization metadata and serializing to temporary json
• beam transform for quantizing node features based on that metadata (calls into quantize_ndarray)
• driver reads the json, constructs FeatureQuantizationMetadata proto, and adds to preprocessed_metadata_pb

Context

• Feature Quantization [1/5]: Compile new protos #733
• Feature Quantization [2/5]: Core Quantization Ops #734
• Feature Quantization [3/5]: Preprocessor (write-side) Updates #735
• Feature Quantization [4/5]: Read updated TFRecords #736
• Feature Quantization [5/5]: Distributed Integration #737

Updated Changelog.md? NO

Ready for code review?: NO

[kmontemayor2-sc]

Nice work, Jacob!

[mkolodner-sc]

Thanks Jacob!

[yliu2-sc]

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