chore(*) clean up ServiceIterator usage

[jpeach]

Summary

Return a ServiceIterator interface rather than a concrete
ServiceItoratorFunc, since the former is the abstraction
that matters.

Full changelog

N/A

Issues resolved

N/A

Documentation

N/A

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

[codecov-commenter]

Codecov Report

Merging #2281 (51f7e26) into master (cc0cb70) will increase coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2281      +/-   ##
==========================================
+ Coverage   51.69%   52.02%   +0.33%     
==========================================
  Files         909      913       +4     
  Lines       41175    41208      +33     
==========================================
+ Hits        21284    21438     +154     
+ Misses      17869    17719     -150     
- Partials     2022     2051      +29     

Impacted Files	Coverage Δ
pkg/core/policy/connection_matcher.go	95.87% <ø> (ø)
api/internal/util/proto/types.go	100.00% <0.00%> (ø)
api/internal/util/proto/proto.go	62.50% <0.00%> (ø)
...kg/plugins/resources/k8s/native/pkg/test/within.go	100.00% <0.00%> (ø)
...k8s/native/controllers/proxytemplate_controller.go	0.00% <0.00%> (ø)
api/mesh/v1alpha1/dataplane_helpers.go	77.84% <0.00%> (+1.70%)	⬆️
pkg/core/resources/manager/cache.go	83.09% <0.00%> (+2.81%)	⬆️
api/mesh/v1alpha1/dataplane_insight_helpers.go	80.24% <0.00%> (+4.93%)	⬆️
pkg/core/bootstrap/autoconfig.go	50.48% <0.00%> (+8.73%)	⬆️
api/observability/v1alpha1/mads.pb.go	23.91% <0.00%> (+9.42%)	⬆️
... and 12 more

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