Iceberg S3-family storage rebuilds the opendal Operator and signer on every file open #6109

What is the problem the feature request solves?

With #6106 every task on an executor shares one FileIO per storage configuration. What that sharing buys depends on what the backend's Storage holds. At the pinned iceberg-rust rev (665c64e), OpenDalStorage::S3, Gcs and Oss hold only the parsed config and, for S3, the access loader. create_operator in iceberg-storage-opendal builds a new opendal Operator on every new_input, new_output, exists and metadata, and opendal-service-s3's build() constructs a new Signer for each operator. The signer is where reqsign keeps the access cache, so with a custom S3 access provider every file open still pays a JNI provide_credential round trip, and every operator re-parses and re-validates its configuration. HTTP connection pooling is unaffected either way: opendal's default transport is process-wide.

For the S3 family, then, #6106 reuses the factory, the config and the bridge, but not the client or signer. The HDFS backend in #5898 already solves this on its side: OpenDalStorage::HdfsNative carries an operator cache keyed by NameNode, so the hdfs-native client is built once per Storage and, with #6106, once per executor.

Describe the potential solution

Apply the HDFS backend's pattern to the S3 family in iceberg-storage-opendal: an operator cache inside OpenDalStorage::S3 (and Gcs, Oss) keyed by the operator's root, so create_operator returns an existing Operator for a bucket it has already built. Since Storage lives as long as the cached FileIO, the signer and its access cache would then be shared by every task on the executor, and the per-open JNI fetch collapses to one per access expiry.

This is an upstream change in apache/iceberg-rust, followed by a pin bump here. Comet cannot do it locally: create_operator is pub(crate) and the Storage trait exposes no operators.

Additional context

Split out of #6105 at review time so it stays open after #6106 merges. The operator cache is only effective once a FileIO outlives a task, which is what #6106 provides.