Add crs for Sen2 subgroups from STAC metadata

[konstntokas]

• Add the CRS information from the STAC metadata stored in the datatree's attributes;
  Temporally fixes the issue https://gitlab.eopf.copernicus.eu/cpm/eopf-cpm/-/issues/932

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
xarray_eopf/amode.py	100.00% <100.00%> (ø)
xarray_eopf/amodes/sentinel2.py	100.00% <100.00%> (ø)
xarray_eopf/amodes/sentinel3.py	100.00% <100.00%> (ø)
xarray_eopf/backend.py	100.00% <100.00%> (ø)
xarray_eopf/version.py	100.00% <100.00%> (ø)

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