Add request cache policy

[yapryntsev]

Idea

Sometimes it is wise not to cache some requests that belong to the same HttpService. It's not possible right now, and as a result, you must create multiply HttpServices with cache hook and without. The idea is that let request decide if the response should be cached.

Solution

Add a new property for the Request protocol, which is then used by the cache hook to determine if the response should be cached. For example:

struct SomeRequest: JsonRequest {
    ...
    var shouldCacheResult: Bool {
        return false
    }
    ...
}

This property will be used in default CachingServiceHook, but you are also free to use this property in your cache-layer implementation