StacAssetRasterSource improvements

[pomadchin]

This issue is to improve StacAssetsRasterSource to use STACItem metadata as much as it is possible. What should such items include? CRS, Extent, Information about layers?

This task is to clean up the RasterSource implementation and to generate an example of the perfect STAC Item. In case some metadata is missing, the StacAssetRasterSource should be able to read it from the STACAsset.