Rename `asset_names` to `variable_names`

[konstntokas]

Pro

• consistent with other xcube data stores (see docuemntation: https://xcube.readthedocs.io/en/latest/dataaccess.html)

Cons

• A STAC item contains multiple assets, and each asset links to a dataset that may include one or more data variables. When the store opens a STAC item, it merges all assets into a single dataset and handles variable naming as follows:
  • Single-variable dataset: The variable is renamed to the asset name. (This is useful for rioxarray, where the variable is often called band_1.)
  • Multi-variable dataset: Each variable is renamed to <asset_name>_<variable_name>.

The idea is that users select data according to the STAC catalog — seeing asset names — but the dataset’s internal variable names don’t always match those asset names. Therefore, these are referred to as asset_names.