Make Cache-Tags header (used for BAN requests) configurable

[lmakarov]

We should make the header used for BAN requests configurable as there seems to be no standard for it.

• These docs on d.o suggest X-Cache-Tags header should be used
• Purger module uses Purge-Cache-Tags header by default
• Acquia Purge uses X-Acquia-Purge-Tags
• Wodby sticks with the Cache-Tags header
• (Symfony FOSHttpCache)[https://foshttpcache.readthedocs.io/en/stable/varnish-configuration.html#ban] uses X-Cache-Tags

Let's call the variable VARNISH_CACHE_TAGS_HEADER and have Cache-Tags (as is now) as the default value.

README should also be updated to mention this usage and configuration of this header.

[sergey-zabolotny]

Fixed in https://github.com/docksal/service-varnish/pull/25