Use pystac-client instead of own implementation

[fwfichtner]

--> https://github.com/stac-utils/pystac-client is much more complete and we should use it instead of our own implementation. I also do not think that we need to wrap it, but better deprecate the whole feature here. Also the data part should be adopted accordingly.

[fwfichtner]

There should be a warning and instructions though following this example: https://www.lesinskis.com/python_deprecation_tutorial.html

[fwfichtner]

@Zajquor implementation should replace stacapi.py and stacapi_io.py.