feat: Make automatic use of Azure storage account keys opt-in

[nameexhaustion]

ref #20634

This makes automatic authentication using inferred storage account keys opt-in using an environment variable to reduce security concerns.

If this environment variable is unset and we observe an error, we print a message to hint to the user that this is an available option:

[CloudOptions::build_azure]: Warning: Azure authentication check failed, subsequent cloud operations may return an error. Note that polars was able to retrieve the storage account key for this URL from the Azure CLI - to allow polars to automatically use storage account keys for authentication, set POLARS_AUTO_AZURE_ACCOUNT_KEY=1 in the environment. To silence this warning, set POLARS_AUTO_AZURE_ACCOUNT_KEY=0.


[codecov]

Codecov Report

Attention: Patch coverage is 0% with 68 lines in your changes missing coverage. Please review.

Project coverage is 78.96%. Comparing base (2e0b3a3) to head (056089f).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-io/src/cloud/options.rs	0.00%	66 Missing ⚠️
py-polars/polars/io/cloud/credential_provider.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20652      +/-   ##
==========================================
- Coverage   79.02%   78.96%   -0.06%     
==========================================
  Files        1557     1557              
  Lines      220551   220727     +176     
  Branches     2514     2513       -1     
==========================================
+ Hits       174283   174301      +18     
- Misses      45695    45852     +157     
- Partials      573      574       +1     

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[nameexhaustion]

Authentication will now prioritize Entra ID (via the Python-side DefaultAzureCredential if azure.identity is installed). Using the storage account key will only happen if we get a permission error (above).