[teleport-update] Display download progress and stats

[sclevine]

This PR logs download progress and stats when teleport-update downloads Teleport tgz files.

The download progress logic is designed to ensure a relatively even distribution of progress log lines over the duration of the download, with a hard maximum of 5 lines.

Example:

ubuntu@legendary-mite:~$ sudo ./teleport-update enable --force-version=16.4.3
2024-12-05T03:22:14Z INFO [UPDATER]   Initiating installation. target_version:16.4.3+Enterprise active_version: agent/updater.go:315
2024-12-05T03:22:16Z INFO [UPDATER]   Downloading Teleport tarball. url:https://cdn.teleport.dev/teleport-ent-v16.4.3-linux-arm64-bin.tar.gz size:163411740 agent/installer.go:322
2024-12-05T03:22:16Z INFO [UPDATER]   teleport-ent-v16.4.3-linux-arm64-bin.tar.gz - progress: 20% agent/logger.go:50
2024-12-05T03:22:17Z INFO [UPDATER]   teleport-ent-v16.4.3-linux-arm64-bin.tar.gz - progress: 40% agent/logger.go:50
2024-12-05T03:22:17Z INFO [UPDATER]   teleport-ent-v16.4.3-linux-arm64-bin.tar.gz - progress: 60% agent/logger.go:50
2024-12-05T03:22:18Z INFO [UPDATER]   teleport-ent-v16.4.3-linux-arm64-bin.tar.gz - progress: 80% agent/logger.go:50
2024-12-05T03:22:19Z INFO [UPDATER]   teleport-ent-v16.4.3-linux-arm64-bin.tar.gz - progress: 100% agent/logger.go:50
2024-12-05T03:22:19Z INFO [UPDATER]   Download complete. duration:3.601365475s size:163411740 agent/installer.go:350
2024-12-05T03:22:21Z INFO [UPDATER]   Extracting Teleport tarball. path:/opt/teleport/default/versions/16.4.3_ent size:648488960 agent/installer.go:370
2024-12-05T03:22:24Z INFO [UPDATER]   Executing new teleport-update binary to update configuration. agent/updater.go:131
2024-12-05T03:22:25Z INFO [UPDATER]   Systemd configuration synced. unit:teleport-update.timer agent/process.go:259
2024-12-05T03:22:25Z INFO [UPDATER]   Service enabled. unit:teleport-update.timer agent/process.go:276
2024-12-05T03:22:25Z INFO [UPDATER]   Finished executing new teleport-update binary. agent/updater.go:137
2024-12-05T03:22:25Z INFO [UPDATER]   Target version successfully installed. target_version:16.4.3+Enterprise agent/updater.go:691
2024-12-05T03:22:25Z INFO [UPDATER]   Gracefully reloaded. unit:teleport.service agent/process.go:110
2024-12-05T03:22:25Z INFO [UPDATER]   Monitoring PID file to detect crashes. unit:teleport.service agent/process.go:113
2024-12-05T03:22:40Z INFO [UPDATER]   Configuration updated. agent/updater.go:331

---

The teleport-update binary will be used to enable, disable, and trigger automatic Teleport agent updates. The new auto-updates system manages a local installation of the cluster-specified version of Teleport stored in /opt/teleport.

RFD: #47126
Goal (internal): https://github.com/gravitational/cloud/issues/10289

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