Detect stale content

### Motivation

To improve response times Delivery API might provide stale content. Stale means that there is a more recent version, but it will become available later. This information is important for developers who implement caching because stale content should have a short TTL. Developers can inspect the value of the **X-Stale-Content** response header. **1** indicates that content is stale. If the header is not present or it has a different value, content is not stale.

### Proposed solution

Extend response models with information whether content is stale. For consistency it is recommended to use a _has stale content_ bool property with the proper case type.

Start by locating the Delivery API response models and reviewing how response headers are represented. Use the X-Stale-Content rules in this issue as the behavior to verify: expose a consistently named boolean on the relevant models, with true only when the header value is 1.

Written by the indexing model from the issue text.