feat(python): Add CredentialProviderAzure parameter to accept user-instantiated azure credential classes

[nameexhaustion]

ref #18931 (comment)

• Closes Allow to explicitly specify Azure TokenCredential in storage_options #20635

Introduces a credentials parameter to CredentialProviderAzure, which can provide a QoL improvement, as users can then do:

pl.scan_parquet(
    "abfss://...@.../...",
    credential_provider=pl.CredentialProviderAzure(
        credentials=DefaultAzureCredential(exclude_managed_identity_credential=True)
    ),
)

instead of:

def credential_provider():
    credential = DefaultAzureCredential(exclude_managed_identity_credential=True)
    token = credential.get_token("https://storage.azure.com/.default")

    return {"bearer_token": token.token}, token.expires_on


pl.scan_parquet(
    "abfss://...@.../...",
    credential_provider=credential_provider,
)

Also adds the above example to the user guide

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.27%. Comparing base (db712e4) to head (65985df).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/io/cloud/credential_provider.py	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21047      +/-   ##
==========================================
- Coverage   79.28%   79.27%   -0.01%     
==========================================
  Files        1583     1583              
  Lines      225549   225554       +5     
  Branches     2586     2588       +2     
==========================================
+ Hits       178816   178818       +2     
- Misses      46143    46146       +3     
  Partials      590      590              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.