feat(sample): implement MIN_(DETECTION|TRACKING)_CONFIDENCE

[homuler]

related to #295

• Port TensorsToDetectionsCalculatorOptions and ThresholdingCalculatorOptions
• Change TensorsToDetectionsCalculatorOptions#MinScoreThresh and ThresholdingCalculatorOptions#Threshold at runtime

To upgrade the installed plugin, you need to remove old scripts first and then build them again.

rm -rf build # or `python build.py clean`
python build.py uninstall
python build.py build ... # build native plugins and scripts again

Notes

1. Mediapipe.Color class is now included, which can conflict with UnityEngine.Color.
2. Currently, Google.Protobuf.dll needs to be compiled manually because the latest version still has a bug (cf. Support MIN_DETECTION_CONFIDENCE and MIN_TRACKING_CONFIDENCE #295 (comment)).