Always set X-Drupal-Cache and X-Drupal-Dynamic-Cache headers, even for responses that are not cacheable

Problem/Motivation

Currently if a response is not cacheable Drupal sets X-Drupal-Dynamic-Cache = UNCACHEABLE, but in some cases it does not. See DynamicPageCacheSubscriber::onResponse for details.

Proposed resolution

Always set X-Drupal-Dynamic-Cache header.

1. This changes the existing X-Drupal-Dynamic-Cache: UNCACHEABLE to X-Drupal-Dynamic-Cache: UNCACHEABLE (poor cacheability).
2. This adds X-Drupal-Dynamic-Cache: UNCACHEABLE (no cacheability) (for responses which aren't instances of CacheableResponseInterface).
3. This adds X-Drupal-Dynamic-Cache: UNCACHEABLE (policy) (when the Dynamic Page Cache request/response policy tagged services deny caching).

This makes it much easier for developers to understand what the Dynamic Page Cache module is doing, and why.

Remaining tasks

None.