add a RasterReader.uniqueValueCount() method

[Andreas Janz]

As the name says, the method returns the unique value counts of a raster band.

Like QgsRasterInterface.bandStatistics() and QgsRasterInterface.cumulativeCut() should it accept a sampleSize parameter to allow for quick estimates.

The proposed methode signature would be:
RasterReader.uniqueValueCount(bandNo: int, extent: QgsRectangle, sampleSize: int = 0, feedback: QgsRasterBlockFeedback = None) -> Dict[float, int]

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