[telemetry] Classify extension install sources

[JeffreyCA]

Background

Existing extension telemetry does not show whether users install extensions from official, preview, local, or custom registries. We want to measure adoption of custom extension sources without collecting source URLs, paths, or user-defined names.

Proposal

Add a privacy-safe extension source category with fixed values:

• azd - https://aka.ms/azd/extensions/registry
• dev - https://aka.ms/azd/extensions/registry/dev
• nightly - https://aka.ms/azd/extensions/registry/nightly
• local
• bundle
• other

Classify sources by their configured location, not their name, since users can assign arbitrary source names. Recognize both known aka.ms URLs and their resolved GitHub URLs.

Emit the category for:

• Extension installs
• Extension upgrades and promotions, including source changes
• Installed-extension inventory
• Extension source registration, if useful for measuring configured sources

Persist enough information with an installed extension to retain its classification if its configured source is later removed or changed. Legacy records that cannot be classified should use unknown.

Acceptance criteria

• Official, dev, nightly, local-file, bundle, and custom sources are categorized correctly.
• No raw source URL, path, host, or user-defined source name is emitted.
• Install, upgrade, promotion, and installed-inventory telemetry expose the applicable category.
• Tests cover known URLs, custom URLs, local sources, bundles, misleading source names, and legacy records.