feat(xds): don't merge listeners of headless service instances

[jijiechen]

Should be part of the cause of issue 9006, but may not be complete.

xref: #9006

Checklist prior to review

• Link to relevant issue as well as docs and UI issues
  • Described
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS
  • Confirmed
• Tests (Unit test, E2E tests, manual test on universal and k8s)
  • Tested using unit tests and manually
  • Don't forget ci/ labels to run additional/fewer tests
• Do you need to update UPGRADE.md?
  • No
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label)
  • No, this is new in 2.6.1

[jakubdyszkiewicz]

I don't think it's a correct fix. You can have multiple tags in outbound in the end. I think we need to compare set of tags in Outbound#Tags

[jakubdyszkiewicz]

Closing for more general fix #9542