14857: prevent cache drop for frontend caches on sitemap generation

[david-fuehr]

Description

Introduces a (dummy) cache tag for frontend in sitemap and robots generation. This prevents the
default and page_cache to be dropped completely. The cache tag for full_page cache is not
modified.

Fixed Issues (if relevant)

1. Sitemap generation cron job flushes entire cache #14857: Sitemap generation cron job flushes entire cache

Manual testing scenarios (*)

1. In Magento admin go to Marketing >> Site Map
2. watch the debug.log
3. Generate sitemap from admin
   1. the sitemap is generated successfully
   2. cache invalidation is logged for "tags":["robots_dummy"] (redis)
   3. cache invalidation is logged for "tagsPattern":"((^|,)robots_1(,|$))"} (varnish)

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @david-fuehr. Thank you for your contribution
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[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-4055 has been created to process this Pull Request

[ghost]

Hi @david-fuehr, thank you for your contribution!
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