Improve 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.

Steps to reproduce

NA

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

Review

User interface changes

NA

API changes

NA

Data model changes

Don't think so

Release notes snippet

Page Cache & Dynamic Page Cache response headers improved to include more detail about the cacheability. See change record.