Implement outdated assets as part of the `/assets` API

[asachs01]

Feature Suggestion

As a Sensu operator, I'd like to see what assets are outdated. While I can do this in sensuctl via the sensuctl asset outdated command I cannot get outdated assets from the Sensu API.

Context

Having an /assets/outdated endpoint would allow for operators to get a list of outdated assets via the API, and develop checks to let them know when an asset is outdated and should be upgraded.

Related #4503