IBX-12173: Added Varnish 9 support for Platform.sh/Upsun

[vidarl]

Question	Answer
JIRA issue	IBX-12173
Type	feature
Target Ibexa version	v4.6
BC breaks	no

Caution

This PR cannot be merged up as-is. Talked to @barw4, and we agree to not merge this into 5.0 and up as it is deprecated and replaced by ibexa/cloud package

Adds .platform/varnish9.vcl and a commented-out varnish:9.0 service block in
.platform/services.yaml, under resources/platformsh/common/4.6/.

Related PRs:

• IBX-12173: Added Varnish 9 VCL http-cache#84 (source of the varnish9.vcl this uses)
• IBX-12173: Added Varnish 9 Docker build docker#64 (matching opt-in Docker build)
• IBX-12173: Added Varnish 9 support for Upsun cloud#12
• IBX-12241: Added Behat coverage for Varnish TTL, stale cache, cache keys and tagging http-cache#85

Checklist:

• Provided PR description.
• Tested the solution manually.
• Provided automated test coverage.
• Checked that target branch is set correctly (4.6, matching the existing
  resources/platformsh/common/4.6/ directory this feature extends).
• Ran PHP CS Fixer for new PHP code. (N/A — no PHP changes)
• Asked for a review (ping @ibexa/engineering).

[bnowak]

Approved to ublock topic, based on it's 1:1 copy between versions.
I don't know vcl much, so if you want to have more confidence, please wait for some more reviewers 😉

[vidarl]

This PR was approved before I added a new commit today, However, that new commit is only a cherry-pick of ibexa/http-cache#86, so I'll skip re-requesting a review