Set X-Drupal-Cache-Max-Age to aid in debugging #cache fun

Problem/Motivation

Debugging caching can be hard.
https://www.previousnext.com.au/blog/ensuring-drupal-8-block-cache-tags-...
...

Although http.response.debug_cacheability_headers: true lets you see your tags and contexts, there's no easy way to see WTF is going on with max-age.

Unfortunately, just knowing what your #cache['max-age'] is doesn't tell you much until this pit of snakes is untangled:

• #2352009: [pp-3] Bubbling of elements' max-age to the page's headers and the page cache
• #2449749: Add #cache['downstream-ttl'] to force expiration after a certain time and fix #cache['max-age'] logic by adding #cache['age']
• #2835068: PageCache caching uncacheable responses (violating HTTP/1.0 spec) + D8 intentionally disabling HTTP/1.0 proxies = WTF
• #2951814: Always set X-Drupal-Cache and X-Drupal-Dynamic-Cache headers, even for responses that are not cacheable
• ...

But, at least knowing you're trying to do the right thing could help in the meanwhile. ;)

It also seems very useful to be able to compare Expires vs. Cache-Control vs. X-Drupal-Cache-Max-Age in various circumstances and configurations as we try to navigate the snake pit.

Proposed resolution

If http.response.debug_cacheability_headers: true is in your services.yml file, also set a X-Drupal-Cache-Max-Age response header.

Remaining tasks

1. Write code
2. Write tests
3. Reviews + refinements
4. RTBC
5. Commit
6. Reach back down into the snake pit and try to make progress on making max-age actually work. :/

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

Probably none.