feat: Add credential provider utility classes for AWS, GCP

[nameexhaustion]

ref #19271 (comment)

• Adds CredentialProviderAWS, CredentialProviderGCP

CredentialProviderAWS can be used to select a different AWS profile:

lf = pl.scan_parquet(
    "s3://...",
    credential_provider=pl.CredentialProviderAWS(profile_name="..."),
)

# `CredentialProviderAWS()` will also pick up `AWS_PROFILE` from the environment
import os
os.environ['AWS_PROFILE'] = 'dummy'

lf = pl.scan_parquet(
    "s3://...",
    credential_provider=pl.CredentialProviderAWS(),
)

Todos:

• In local testing, GCP is not being cached properly

[nameexhaustion]

Here are the differences with enabling the "auto" behavior compared to existing:

• If boto3 is installed, the Python-side CredentialProviderAWS will be used by default for S3 URLs. CredentialProviderAWS introduces a new effect of respecting the AWS_PROFILE environment variable if it is set, otherwise it will behave identically to what we currently already do.
• If google-auth is installed, the Python-side CredentialProviderGCP will be used by default for GS URLs. It behaves identically to what object-store already does, except in the error case the message is slightly more informative:
  • (CredentialProviderGCP) google.auth.exceptions.DefaultCredentialsError: Your default credentials were not found. To set up Application Default Credentials, see https://cloud.google.com/docs/authentication/external/set-up-adc for more information.
  • (object-store (existing)) polars.exceptions.ComputeError: Generic GCS error: Error performing token request: Error after 2 retries in 3.251593125s, max_retries:2, retry_timeout:10s, source:error sending request for url (http://169.254.169.254/computeMetadata/v1/instance/service-accounts/default/token?audience=https%3A%2F%2Fwww.googleapis.com%2Foauth2%2Fv4%2Ftoken)

[nameexhaustion]

Also added to the Python docs, but everything has been marked unstable

[ritchie46]

Nice, we should quickly setup an azure env as well.