Add IsCached boolean to resources

[wwwlicious]

The goal is to detect which responses will be cached.
The simple implementation would be to detect any usage of the [CacheResponse] attribute on the endpoint.

Not sure if Request.ToOptimisedResult or customised caching can be reliably added or if there aspects of the CacheFeature plugin that can be added also. (perhaps by looking up a key convention on a cache dictionary)