feat(kuma-cp): add gateway support for external services (backport #2990)

[mergify]

This is an automatic backport of pull request #2990 done by Mergify.
Cherry-pick of ad2e07d has failed:

On branch mergify/bp/release-1.3/pr-2990
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Changes to be committed:
	new file:   pkg/plugins/runtime/gateway/cluster_generator.go
	modified:   pkg/plugins/runtime/gateway/generator.go
	new file:   pkg/plugins/runtime/gateway/match/external.go
	new file:   pkg/plugins/runtime/gateway/testdata/18-gateway-route.yaml
	new file:   pkg/plugins/runtime/gateway/testdata/19-gateway-route.yaml
	modified:   pkg/xds/topology/outbound.go
	modified:   test/e2e/gateway/gateway_universal.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   pkg/plugins/runtime/gateway/gateway_route_generator_test.go
	both modified:   pkg/plugins/runtime/gateway/plugin.go
	both modified:   pkg/plugins/runtime/gateway/route_table_generator.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[jpeach]

Has conflicts because #2954 didn't get merged (because it had conflicts too).

[jpeach]

Has conflicts because #2954 didn't get merged (because it had conflicts too).

Closing due to missing dependencies