fix(egress): same external service tag in multiple meshes (backport of #11667)

[kumahq]

Automatic cherry-pick of #11667 for branch release-2.6

Generated by action

cherry-picked commit 4e5e814
⚠️ ⚠️ ⚠️ Conflicts happened when cherry-picking! ⚠️ ⚠️ ⚠️

On branch release-2.6
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You are currently cherry-picking commit 4e5e814f7.
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Changes to be committed:
	modified:   pkg/xds/generator/egress/generator_test.go
	modified:   pkg/xds/generator/egress/testdata/01.externalservice-only.golden.yaml
	modified:   pkg/xds/generator/egress/testdata/03.mixed-services.golden.yaml
	modified:   pkg/xds/generator/egress/testdata/04.mixed-services-custom-trafficroute.golden.yaml
	modified:   pkg/xds/generator/egress/testdata/05.mixed-services-with-custom-trafficpermissions.golden.yaml
	modified:   pkg/xds/generator/egress/testdata/06.mixed-services-with-external-in-other-zone.golden.yaml
	new file:   pkg/xds/generator/egress/testdata/input/same-kuma-io-service.yaml
	new file:   pkg/xds/generator/egress/testdata/same-kuma-io-service.golden.yaml
	modified:   pkg/xds/generator/egress/testdata/subsets-with-external-meshhttproute.golden.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   pkg/xds/generator/egress/external_services_generator.go