Create an exclusion mask for raster classification

[gilbertocamara]

Describe the requested improvement
When classifying raster data cubes, it is useful to provide an exclusion mask that will remove certain areas from the classified cube. This situation occurs when working with deforestation monitoring, when users want to exclude past deforestation and only focus on the yearly increments.

Associated sits API function
The sits_classify.raster_cube() function will have an additional parameter called exclusion_mask which will be a multipolygon object.

Additional context
Consider the use of GDAL to do this operation.