Added write support for GDALRaster

- Instantiation of GDALRaster instances from dict or json data.
- Retrieve and write pixel values in GDALBand objects.
- Support for the GDALFlushCache in gdal C prototypes
- Added private flush method to GDALRaster to make sure all
  data is written to files when file-based rasters are changed.
- Replaced ``ptr`` with ``_ptr`` for internal ptr variable

Refs #23804. Thanks Claude Paroz and Tim Graham for the reviews.