GEP-2907: TLS mode and allowed routes

[mlavacca]

What type of PR is this?

/kind gep

What this PR does / why we need it:

This PR updates GEP-2907 with two different aspects:

• definition of TLS mode (Terminate and Passthrough)
• what routes can be attached to Listeners that specify TLS configuration.

This PR intends to reach an agreement that will make #2111 and #1474 addressable.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[mlavacca]

/cc @candita

[mlavacca]

I have no idea why the CI robot has put this in hold 🤔

/unhold

[candita]

Is passthrough mode on a TCPRoute based on what we see in

gateway-api/apis/v1/gateway_types.go

Line 297 in e971a8d

TLS *GatewayTLSConfig `json:"tls,omitempty"`

, or something else?

[candita]

E.g. it says It is invalid to set this field if the Protocol field is "HTTP", "TCP", or "UDP".

[mlavacca]

It was based on a discussion we had about enabling TLS on TCPRoute. But I think that at least for the time being, even considering the high level of uncertainty the L4 routes have at the moment, we should be aligned with the current API specs, without aiming at improving it.

[mlavacca]

/hold

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[mlavacca]

/retest

[youngnick]

We might need a note here that for TCPRoute, the packets are passed through untouched to the backend. If you want to terminate a TLS stream at a Gateway and forward a TCP stream unencrypted to the backend, use TLSRoute.