Consistent GRIB handle usage in field

[sandorkertesz]

Description

This PR implements the following changes for fields generated from GRIB messages:

• when the metadata is changed with set() we cannot access any raw (GRIB) metadata on the new field. The reason for this is that the original GRIB message/handle is not updated when set() is called and it goes out of sync with the updated components of the field. Note: previously the (potentially out-of-sync) GRIB metadata was available on the modified field.
• The raw (GRIB) metadata can be brought in sync with the field components by calling the newly added sync() method.

        >>> import earthkit.data as ekd
        >>> fl = ekd.from_source("sample", "test.grib").to_fieldlist()
        >>> f = fl[0]
        >>> f1 = f.set({"parameter.variable": "msl", "parameter.units": "Pa"})
        >>> f1.get("metadata.shortName")
        None
        >>> f1.metadata("shortName")
        KeyError: 'metadata.shortName' not found in field
        >>> f1.message()
        None
        >>> f2 = f1.sync()
        >>> f2.get("metadata.shortName")
        'msl'
        >>> f2.metadata("shortName")
        'msl'
        >>> f2.message()
        b'GRIB\x00\x ....

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.52778% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.64%. Comparing base (5eb79ea) to head (216b864).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/encoders/test_grib_encoder.py	93.93%	3 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #968      +/-   ##
===========================================
+ Coverage    81.26%   81.64%   +0.37%     
===========================================
  Files          223      226       +3     
  Lines        14624    14980     +356     
  Branches       718      745      +27     
===========================================
+ Hits         11884    12230     +346     
- Misses        2521     2524       +3     
- Partials       219      226       +7     

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