Fix QuotaSpec crash

[howardjohn]

Fixes #24264

I have no strong confidence this works, as we clearly have no end to end tests or this would have been caught. Apparently no one has used QuotaSpec with istiod since this should be broken since 1.5.0

Basically the issue is we are using the golang proto struct instead of gogo proto struct.

[istio-policy-bot]

🤔 🐛 You appear to be fixing a bug in Go code, yet your PR doesn't include updates to any test files. Did you forget to add a test?

Courtesy of your friendly test nag.

[istio-testing]

@howardjohn: The following tests failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
lint_istio	5fdf235	link	/test lint_istio
gencheck_istio	5fdf235	link	/test gencheck_istio
unit-tests_istio	5fdf235	link	/test unit-tests_istio

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[gargnupur]

@howardjohn : I thought we hade ratelimiting tests running in post submit?
We would have to change mixer.go too.. pilot/pkg/networking/plugin/mixer/mixer.go.. that's using mpb "istio.io/istio/pilot/pkg/networking/plugin/mixer/client"....

[howardjohn]

@gargnupur the problem is the CRD reading library only supports gogo proto and XDS only supports golang protobuf. We run pure envoy tests for rate limitting but none that go through the full pilot pipeline