Update Auth GEP with Implementable details

[youngnick]

What type of PR is this? /kind gep

What this PR does / why we need it: This adds the design rationale and API design for phase 1 of the Auth GEP, adding a Filter to HTTPRoute.

Which issue(s) this PR fixes:

Updates #1494

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: youngnick

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• ~~geps/OWNERS~~ [youngnick]

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[kflynn]

After @howardjohn's entirely appropriate gentle mockery for the lack of proper specification for this protocol, I've abused the GEP machinery a bit to get the doc off my laptop into a state and place where at least other folks can read it: https://github.com/kubernetes-sigs/gateway-api/pull/3892 (or https://deploy-preview-3892--kubernetes-sigs-gateway-api.netlify.app/geps/gep-9999/ if you just want the formatted version).

(As stated in that document, I don't think that Gateway API is actually the correct place for this -- it was just a low-friction way to let others see it.)

[youngnick]

Okay, reviewing this, I think I've answered all the outstanding things, it seems that this is ready to move to Implementable, so I'll remove the hold and wait for an LGTM from someone. Next steps will be for me to actually add the APIs to the Go types and then move to Experimental. But I can do that after 22 July.

/unhold

[robscott]

Thanks @youngnick! Will leave hold to give you a chance to reply to @shaneutt's comment above.

/lgtm /hold

[youngnick]

/unhold