Delivery Format: set `kn-ingress-format` ce extension on imc ingress

[Cali0707]

Problem To support dispatching events from the InMemoryChannel with the correct format when a user sets the spec.delivery.format to ingress, we need to set a cloudevent extension of kn-ingress-format to specify which format was used. This should be set to structured if the input event was structured, and binary if the input event was in binary. However, since this event may have been forwarded from earlier ingress, we need to only set the kn-ingress-format extension if it is not already present on the event.

Persona: Which persona is this feature for?

Exit Criteria Unit tests verifying that:

1. If the extension is not present it is set correctly for both structured and binary events
2. If the extension is already present, it is not changed for both structured and binary events

Time Estimate (optional): How many developer-days do you think this may take to resolve? 1-2

Additional context (optional) Add any other context about the feature request here.

• The parent issue is: https://github.com/knative/eventing/issues/8057
• The channel ingress handler code is here: https://github.com/knative/eventing/blob/5f6713a5dcb593f3930f6e974a4f3c1153fa799a/pkg/channel/event_receiver.go#L177-L278

[Cali0707]

/good-first-issue /triage accepted

[knative-prow[bot]]

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In response to [this](https://github.com/knative/eventing/issues/8076): >/good-first-issue >/triage accepted Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[EraKin575]

/assign

[EraKin575]

/unassign

[Cali0707]

/close