Added RasterSource/GDALBand GDAL objects

Based on Daniel Wiesmann's raster branch. Refs #23804.
For geometries, the data-source is different from a Geometry instance, as a data-source represents a list/table of geometries and attributes. This difference does not exist for rasters, the data source *is* the raster. Thats why I named this class GDALRaster, as I suppose that is the more intuitive name. Did you have a second class in mind for the actual raster? If not, all rasters will be instances of RasterSource, which I think is not ideal.