updated to check a wave-fc file and check for valid kerchunk indices

[lawi119]

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Description:
Added in an update tests/test_stac.py that checked for valid kerchunk indices and used a wave-fc file for the checks. This file was meant to be a part of the previous PR.

PR checklist:

• [Y] Code is formatted (run scripts/format).
• [Y] Code lints properly (run scripts/lint).
• [Y] Tests pass (run scripts/test).
• [Y] Documentation has been updated to reflect changes, if applicable.
• [Y] Changes are added to the CHANGELOG.

[TomAugspurger]

A couple test failures:

>           assert item.properties["kerchunk_indices"] == kerchunk_indices

I think we changed this to kerchunk:indices.

E               FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/work/ecmwf-forecast/ecmwf-forecast/ecmwf/20231019/00z/0p4-beta/wave/20231019000000-0h-wave-fc.grib2'

I think now we need the files to be present, since we actually read them (previously we just used the path structure).

I'd prefer not to have large data files in the source repo. So we'll want to fetch these (probably from Azure Blob Storage) when we run the test. We can cache it locally so that we don't need to download it every time. Can you take care of that?

[lawi119]

Done.

[TomAugspurger]

Looks like one test failure, from the updated base64 logic:

E             -           'step/0': '\x00\x00\x00\x00\x00\x00\x00\x00',
E             +           'step/0': 'base64:AAAAAAAAAAA=',