Allow internal callers to inject RouteGenerator

[nareddyt]

Bubble up routeGens parameter to MakeListener, which we call internally.

Also allows us to remove ServiceInfo from MakeListener 💯 In follow-up CL, I will completely delete ServiceInfo.

[google-oss-prow]

@nareddyt: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ESPv2-presubmit-tsan	00bb51e	link	true	/test ESPv2-presubmit-tsan
ESPv2-anthos-cloud-run-e2e-anthos-cloud-run-http-bookstore	00bb51e	link	true	/test ESPv2-anthos-cloud-run-e2e-anthos-cloud-run-http-bookstore
ESPv2-presubmit-asan	00bb51e	link	true	/test ESPv2-presubmit-asan

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[google-oss-prow]

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