Create NPEP-127 for external ingress endpoints

[npinaeva]

An attempt to collect use cases for external ingress endpoints.
Reviews and more examples are welcome.

Tracking issue #127

Previous PR that was closed: #128

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[k8s-ci-robot]

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

Test name	Commit	Details	Required	Rerun command
pull-network-policy-api-verify	0a30132	link	true	/test pull-network-policy-api-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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