Fix crash when getting the gridSpec GRIB metadata key

[sandorkertesz]

With ecCodes 2.41.0 when we get the gridSpec key with a default value an exception not caught in the high-level ecCodes Python API is raised.

ds[0].metadata("gridSpec", default=None)
...

... python3.11/site-packages/eccodes/highlevel/message.py:68: in get
    return self._get(name, ktype=ktype)

 gribapi.errors.FunctionNotImplementedError: Function not yet implemented

Previously, the code above returned None, because ecCodes raised a different exception that was handled in the high level ecCodes Python API. It is already fixed in eccodes-python "develop" but not yet released (ecmwf/eccodes-python@5ce7762)

This PR provides a fix for it in the earthkit-data GRIB interface ensuring that:

ds[0].metadata("gridSpec", default=None)

works with all ecCodes versions.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.53%. Comparing base (66cddef) to head (8c1394d).
⚠️ Report is 257 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #719      +/-   ##
==========================================
+ Coverage   90.02%   90.53%   +0.50%     
==========================================
  Files         158      164       +6     
  Lines       12086    12458     +372     
  Branches      593      609      +16     
==========================================
+ Hits        10881    11279     +398     
+ Misses       1023      998      -25     
+ Partials      182      181       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.