Add support for caching HTTP requests

[Enngage]

Motivation

Reduce the amount of HTTP requests made by application & increase app's responsiveness

Proposed solution

There are 3 ways to accomplish this:

1. Have cache as a part of kentico-cloud-delivery package
2. Have cache as part of kentico-cloud-core package
3. Create new HttpService which incorporates cache and let packages register it instead of default one

Additional context

There should be a way of 'clearing' cache items based on set of keys - e.g. "clear all items of 'article' type", "clear item with codename 'x'"...

Devs should be able to configure how long items are stored in cache by default or per query (this might require caching logic to by part of Delivery SDK)