Allow using high level field metadata keys in grib encoder

[sandorkertesz]

Description

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 99.21260% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.46%. Comparing base (ab2310b) to head (94e406e).

Files with missing lines	Patch %	Lines
tests/encoders/test_grib_encoder.py	99.21%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #997      +/-   ##
===========================================
+ Coverage    81.32%   81.46%   +0.14%     
===========================================
  Files          236      236              
  Lines        15308    15432     +124     
  Branches       751      754       +3     
===========================================
+ Hits         12449    12572     +123     
  Misses        2634     2634              
- Partials       225      226       +1     

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