Feature/improve vcl backend response

[toonvd]

Description

This PR is a rework of the Magento VCL (the vcl_backend_response part) PR found here:
https://github.com/magento/magento2/pull/36796/files

All credits go to @peterjaap and @ThijsFeryn, I am simpling slicing this up and removing controversial parts to get early approval.

Related Pull Requests

#62
#69

Manual testing scenarios

• Load the new VCL
• Test if cacheable content is still cached

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)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)