feat(python): Experimental credential provider support for Delta read/scan/write

[nameexhaustion]

Adds automatic Python-side credential provider initialization to the Delta I/O functions (read_delta, scan_delta, write_delta), similar to those previously added to other I/O functions. This includes DefaultAzureCredential (ref #20808):

• Support for DefaultAzureCredential with pl.read/scan_delta() #20808

Before calling into the Delta Python library, we directly call the credential provider on the Python-side and inject them into storage_options, as the Delta Python library does not accept a callable credential provider. This works as our built-in credential providers (AWS, Azure, GCP) all return object_store-compatible key-value pairs - i.e. it's valid to do something like this:

creds, _expiry = pl.CredentialProviderAzure()()

pl.read_delta(
    "abfss://...",
    storage_options=creds,
)

[ion-elgreco]

Just fyi, inside delta-rs we have this for AWS actually

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 24 lines in your changes missing coverage. Please review.

Project coverage is 79.64%. Comparing base (b333ab7) to head (240aab3).
Report is 43 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/io/cloud/credential_provider.py	25.00%	12 Missing ⚠️
py-polars/polars/dataframe/frame.py	36.36%	5 Missing and 2 partials ⚠️
py-polars/polars/io/delta.py	58.33%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20842      +/-   ##
==========================================
- Coverage   79.64%   79.64%   -0.01%     
==========================================
  Files        1571     1571              
  Lines      223165   223200      +35     
  Branches     2546     2554       +8     
==========================================
+ Hits       177737   177757      +20     
- Misses      44844    44855      +11     
- Partials      584      588       +4     

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[nameexhaustion]

Just fyi, inside delta-rs we have this for AWS actually

Nice! We unfortunately aren't able to use it at the moment as we don't have rust-level integration with delta.