Consolidating client and retry logic into StorageClient.

Currently, the plugin uses an executor class and retry logic from performUploadWithRetry and RetryStorageOperation. The plugin should be refactored to use the StorageClient to have consistent logic with the other Jenkins GCP plugins.

Start by tracing the executor class, performUploadWithRetry, and RetryStorageOperation, then compare their behavior with StorageClient and the corresponding logic in other Jenkins GCP plugins. Consolidate the client and retry responsibilities so the plugin uses StorageClient consistently; done means the old paths are no longer used and uploads retain their retry behavior.

Written by the indexing model from the issue text.