Add function to retrieve requested variables from data request API

[rbeucher]

Add utility function to get the list of required variable for a CMIP7 experiment, based on priority level.

Utility for #227

[codecov]

Codecov Report

❌ Patch coverage is 8.33333% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.58%. Comparing base (7549b46) to head (110c936).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/access_moppy/utilities.py	8.33%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
+ Coverage   47.47%   47.58%   +0.10%     
==========================================
  Files          22       22              
  Lines        4183     4195      +12     
==========================================
+ Hits         1986     1996      +10     
- Misses       2197     2199       +2     

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