[ZFDB] feat: Initial version of custom store builder

[tbkr]

Description

Now we are able to specify arbitrary subgroups and access the data from there.

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.

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/fdb/pull-requests/PR-282

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.05%. Comparing base (abb281c) to head (26e96b7).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #282      +/-   ##
===========================================
- Coverage    71.10%   71.05%   -0.05%     
===========================================
  Files          371      371              
  Lines        23486    23486              
  Branches      2465     2462       -3     
===========================================
- Hits         16699    16689      -10     
- Misses        6787     6797      +10     

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