Add Datacube STAC extension

[JohanKJSchreurs]

We already support the EO (Electro-Optical) extension and the Raster Extension to provide information about the bands in the assets, but there was a demand for the DataCube extension.

The EO and Raster extension cover different things about rasters but the DataCube seems to give a more unified set of data we need about the bands.

Datacube extension: https://github.com/stac-extensions/datacube

List of well-known extensions: https://stac-extensions.github.io/

[jdries]

This one is simple, but fairly important because the python client relies on this extension, for instance to determine if the cube has a time dimension.

[EmileSonneveld]

This would also be needed to get pixel perfect alignment between source and output data Open-EO/openeo-geopyspark-driver#648
And the step size could be used to check if a batch job request will be too large