Further decouple CubeMetadata from CollectionMetadata, and properly apply spatio-temporal filtering

supporting issue for #827 and related

PR #541 (for issue #464) introduced CubeMetadata as a less confusing alternative to CollectionMetadata, to focus on the concrete metadata of a cube, instead of overall metadata of a whole collection.
But we're not there yet: CollectionMetadata is still too sticky/dominant.

for example:

cube = connection.load_collection(
    "SENTINEL2_L2A",
    spatial_extent={"west": 3, "south": 51, "east": 3.1, "north": 51.1},
    temporal_extent="2026-09",
    bands=["B03"]
)

While cube is a tiny slice of the whole SENTINEL2_L2A data set, cube.metadata still point directly to SENTINEL2_L2A metadata, e.g. visualizing cube.metadata will show the full metadata of SENTINEL2_L2A: all bands, whole world, whole time converage.

e.g.

>>> cube.metadata.temporal_dimension.extent
['2015-06-27T10:25:31Z', None]
>>> cube.metadata.spatial_dimensions[0].extent
[-180, 180]

Band handling works better however: cube.metadata.band_dimension.band_names shows just the selected band ['B03'], but the default visualization of the whole metadata object will show all bands

Next steps: further decouple CubeMetadata and CollectionMetadata

at least: load_collection and load_stac should zone in on CubeMetadata as soon as possible, instead of confusingly still attaching CollectionMetadata
possible/necessary to eliminate the inheritance relation (class CollectionMetadata(CubeMetadata))?

problem: the apply_metadata UDF signature is defined in terms of CollectionMetadata instead of CubeMetadata (which would be more correct):

openeo-python-client/openeo/udf/udf_signatures.py

Line 82 in 71bf07a

    
           def apply_metadata(metadata: CollectionMetadata, context: dict) -> CollectionMetadata:

to be investigated how easy this signature can be updated in this context without breaking existing UDF handling

Make openeo.metadata a subpackage

to be investigated how easy this signature can be updated in this context without breaking existing UDF handling

the UDF detection logic in openeo-geopyspark only looks at function name apply_metadata and ignores the exact argument signatures, so it's safe to change the signature here

fixup! Issue #949 promote CubeMetadata usage

Issue #949 promote CubeMetadata usage

at least: load_collection and load_stac should zone in on CubeMetadata as soon as possible, instead of confusingly still attaching CollectionMetadata

Already handled that with 43d53d6

WIP attempt to shoehorn filter_temporal in TermporalDimension

Handle filter_temporal in CubeMetadata down to TermporalDimension

Handle filter_temporal in CubeMetadata, down to TermporalDimension

Reorganise test_metadata.py

Basic handling of filter_bbox in CubeMetadata, down to SpatialDimension