Extensions for gateway

[daixiang0]

What would you like to be added:

Add Extension API for gateway.

Why this is needed:

Optimize ease of use, flexibility and scalability.

Now every gateway implement has its own unique features while Kubernetes Gateway API cannot cover them.
It is ok that users use Gateway API + specific gateway API, but would be better if only use Gateway API.

In addition to ease of use issues, more flexibility would be introduced by Extension API. As we know, bring a new feature into Gateway API usually speed much time, During this time, Extension API can help as well.

Full design is as https://docs.google.com/document/d/1FjUEKtpymVrY0xAyZVz1nnVB1fryHU7s9XWE6C3Bsww/edit?usp=sharing

[arkodg]

@daixiang0 Gateway API already supports extending the API using PolicyAttachments https://gateway-api.sigs.k8s.io/geps/gep-713/

[daixiang0]

I knew it but it can not support some features like this feature in Kong.

Those features are not in policy field but implemented in the gateway.

[spacewander]

The gateway can define its own policy. For example, HTNN defines the FilterPolicy to run plugins: https://github.com/mosn/htnn/blob/main/site/content/en/docs/concept/filterpolicy.md

[daixiang0]

The gateway can define its own policy. For example, HTNN defines the FilterPolicy to run plugins: https://github.com/mosn/htnn/blob/main/site/content/en/docs/concept/filterpolicy.md

I am confused about those belongs, now we have GEP, but for example, HTNN still use htnn.mosn.io not gateway.networking.k8s.io, I can't think it followed GEP standard from its YAML statement though in fact it did.

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