Config observers run synchronously in a reactor turn and must not
suspend. Split the previous monolithic async update_config() coroutine
into two phases:
Sync (runs in the observer, never suspends):
- S3: atomically swap _cfg (lw_shared_ptr) and set a credentials
refresh flag.
- GCS: install a freshly constructed client; stash the old one for
async cleanup.
- storage_manager: update _object_storage_endpoints and fire the
async cleanup via a gate-guarded background fiber.
Async (gate-guarded background fiber):
- S3: acquire _creds_sem, invalidate and rearm credentials only if
the refresh flag is set.
- GCS: drain and close stashed old clients.
Add a copy_object method to the object_storage_client
interface for server-side object copies, with
implementations for both S3 and GCS wrappers.
The S3 wrapper delegates to s3::client::copy_object.
The GCS wrapper delegates to gcp::storage::client's
cross-bucket copy_object overload.
This is a prerequisite for implementing sstable clone
on object storage.
Add an `exists` method to the storage abstraction to allow S3, GCS,
and local storage implementations to check whether an sstable
component is present.
