Expose the running Feast version in the UI #6842

What is the problem?

The Feast UI does not show which Feast package version the running service is using. The UI server health endpoint returns status only, so operators need container or cluster access to verify a deployed release.

Proposed behavior

Add an authenticated GET /api/v1/version endpoint to the shared registry REST API and show its result in the UI sidebar. The response should be derived from feast.version.get_version() in the running process, for example:

{"feast_version": "0.66.0"}

The UI should hide the label without disrupting navigation when the endpoint is unavailable, preserving compatibility with older or custom backends. The existing health endpoint should remain unchanged.

Why use the shared REST API?

Both the UI-mounted API and standalone registry REST server register the same router set and apply authentication at the application level. A shared route provides one contract without duplicating authentication or server-specific code.

Acceptance criteria

• The value comes from installed package metadata in the running Python process.
• Existing REST authentication applies to the endpoint.
• The UI displays the version in its sidebar when available.
• Missing or failed version requests are nonfatal.
• Health endpoint behavior remains unchanged.