Notify users in azd CLI when installed extensions have updates available, like an "outdated" notification message

[jongio]

Problem

Users can install azd extensions, but currently there is no CLI notification when a locally installed extension has a newer released version available. This can lead to users running older extension versions and missing important improvements or security fixes.

Proposal

Add a notification feature to the azd CLI:

• On startup or after running extension-related commands, print a banner or message when an installed extension has an update available, similar to the azd outdated message for the core tool.
• Example: "Extensions with updates available: my-ext (0.1.2 → 0.2.0). Run azd extension update <name> to upgrade."
• Optionally add a command azd extension outdated or azd extension check-updates to explicitly list out-of-date extensions and their latest versions.
• Allow azd extension update --all to update all outdated extensions in one go.

Implementation notes

• Perform a version check against the extension registry or gallery, comparing local installed versions to the latest available.
• Limit network calls and cache results to be efficient and avoid blocking CLI startup.
• Don't block CLI execution if the update check fails due to network issues; notification should be non-blocking.
• Add documentation for the notification feature and update privacy/telemetry info if required.

Acceptance criteria

• CLI notifies user interactively when installed extensions have updates available.
• azd extension outdated (or similar) command lists installed extensions and shows if updates are available.
• All update checks are efficient, rate-limited, and do not block CLI usage.

Additional context

Many users miss extension updates unless they manually check for them. Adding a notification improves user experience, extension reliability, and security.

[JeffreyCA]

@kristenwomack @puicchan could we consider this for the upcoming sprint? I second this: "Many users miss extension updates unless they manually check for them. Adding a notification improves user experience, extension reliability, and security." And I don't think a many users would know to run azd ext upgrade, especially if they're unfamiliar with extension commands or they went through the auto-install flow.

Allow azd extension update --all to update all outdated extensions in one go.

This is supported today through azd extension upgrade --all.

Optionally add a command azd extension outdated or azd extension check-updates to explicitly list out-of-date extensions and their latest versions.

azd extension list already exists today and supports the --installed flag, so if we follow the existing pattern adding an --outdated flag could be another approach.

[spboyer]

Related to #5965