load_stac: avoid dependency on eo:bands

[jdries]

When loading our own netcdf metadata, I get:
No band assets found in items; a band asset requires an "eo:bands" property with a "name".
While the use of the name in eo:bands is relevant, all stac assets also have a default name, because assets in STAC are represented by a dictionary.
Can we fall back to those names, if eo:bands is not present? I think that would make our load_stac less extension dependent.

[bossie]

Yes, I've been considering this as a best-effort approach. The implicit assumption then is that those are single band assets.

[soxofaan]

Maybe worth mentioning that in the python client we're also started to collect functionality to parse band information from stac:

• No metadata for datacube loaded with load_stac openeo-python-client#527
• Issue527 load stac metadata openeo-python-client#548
• load_stac: get asset metadata from "item_assets" in collection instead of from items openeo-python-client#553

We might want to consolidate efforts here