[packaging] Make polars installation optional

[vkarak]

This PR introduces an extra called no-analytics that if passed, it will skip the polars dependency. However, this dependency is needed if RFM_ENABLE_RESULTS_STORAGE is set. We expand the validation check in the CLI for this. Also to avoid failures in lightweight installations, polars is only imported in the functions that is needed.

Finally, this PR introduces also documentation improvements.

Future note: polars will likely be completely removed from dependencies.

Closes #3676.

[vkarak]

@dmargala With this you will be able to pip install reframe-hpc[no-analytics] and it will not install polars.

[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.90%. Comparing base (58b83f2) to head (6aad161).

Files with missing lines	Patch %	Lines
reframe/frontend/cli.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3684      +/-   ##
==========================================
- Coverage   91.91%   91.90%   -0.02%     
==========================================
  Files          62       62              
  Lines       13772    13779       +7     
==========================================
+ Hits        12659    12663       +4     
- Misses       1113     1116       +3     

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[vkarak]

Converting it back to draft; trying to understand why the CI doesn't pick polars as a dependency although it should do it by default.

[vkarak]

It seems the uv sync --group dev done to run unit tests removes polars, thus the errors.