StorageControl and idempotency

Some of the storage control operations can use client-assigned request ids to become idempotent. Basically the client library assigns a UUID to the request, and retries it, the server can detect if it is a dup and return the previous response.

For the other requests the client can include the `x-goog-gcs-idempotency-token` header for a similar effect. Googlers can see go/gcs-rust-sdk-idempotency-design for a possible implementation using sidekick and a bit of hand-crafted code.

Start by locating the Swift StorageControl client entry points and its request and retry handling. Compare operations that accept client-assigned request IDs with those requiring the x-goog-gcs-idempotency-token header, using the referenced idempotency design for context; done means supported retries are detected as duplicates and return the previous response.

Written by the indexing model from the issue text.