Automatic merge from submit-queue (batch tested with PRs 47302, 47389, 47402, 47468, 47459)
[GCE] Fix ILB sharing and GC
Fixes#47092
- Users must opt-in for sharing backend services (alpha feature - may be removed in future release)
- Shared backend services use a hash for determining similarity via settings (so far, only sessionaffinity) (again, this may be removed)
- Move resource cleanup to after the ILB setup.
/assign @bowei
**Release note**:
```release-note
NONE
```
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