Manual trigger mode for bundle refresh

BundleDownloader always polls (startPolling schedules an immediate fetch and chains jittered re-polls). OPA Go supports trigger: manual so callers can refresh on demand — useful for tests, for hosts that fetch via an external scheduler, and for environments where polling intervals don't fit the deployment model.

• Add a trigger mode to BundleConfig (periodic default, manual).
• In manual mode, skip the scheduler and expose a refresh() method on the per-bundle downloader.
• BundlePlugin still completes the initial-activation future, but only when the first manual refresh() succeeds (or the plugin is started with no initial fetch — decide which).
• Validation: reject polling config under manual trigger.
• Tests covering manual refresh and invalid combinations.