S3 credential SPI: per-location credentials within a bucket #6207

Problem. On the Parquet path, object_store::CredentialProvider::get_credential() receives no request path, so one S3 store presents one credential and Comet caches one store per bucket and configuration. A CometS3CredentialProvider gets one credential per bucket, requested with the path of the first file read. Deployments whose policies differ by location within a bucket, for example one STS session for warehouse/sales and another for warehouse/finance, get 403s everywhere but the first location.

Proposal. Add an opt-in @Public extension:

public interface CometS3LocationScopedCredentialProvider extends CometS3CredentialProvider {
  List<String> getPolicyLocations(String bucket) throws Exception;
}

The provider lists every location in the bucket that has its own policy. Comet serves each request with the credential of the longest location covering its path, matched one segment at a time, with the bucket root as an implicit location. It requests a location's credential by calling getCredentialsForPath with the location as the path. Locations apply to native Parquet reads; the Iceberg path is unchanged. Providers that implement only the base interface are unaffected.

Compatibility. The change is additive: a new interface with no change to existing types. Under the versioning policy, adding an abstract method to it later, or changing how paths match locations, would require a major release.

Implementation: #6031.