Use earthkit_grid_spec in generated Xarray

[sandorkertesz]

Description

Changed the earthkit grid spec attribute in the Xarray generated by the Xarray engine from "ek_grid_spec" to "earthkit_grid_spec".

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 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.77%. Comparing base (6f73284) to head (4e7f0ed).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/xr_engine/test_xr_engine_attrs.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #979   +/-   ##
========================================
  Coverage    81.77%   81.77%           
========================================
  Files          227      227           
  Lines        15082    15082           
  Branches       746      746           
========================================
  Hits         12334    12334           
  Misses        2523     2523           
  Partials       225      225           

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