Add support for Knative

[tombanksme]

This is a proof-of-concept pull request to add the canary release deployment strategy for Knative (#903). It's far from production ready but I wanted to see if there's appetite for this feature before I invest more time into completing it.

Currently Flagger can target a Knative service & complete the canary release process. I've not tested rollbacks yet although I think it should just work. The pull request needs some extra work to add test coverage; throw errors when using unsupported release processes & add Knative specific Kubernetes events.

Let me know what you think!

Example

The following canary will target a Knative Service. Once the canary has been initialised you can start the canary release process by creating a new revision of the service.

apiVersion: flagger.app/v1beta1
kind: Canary
metadata:
  name: example
spec:
  targetRef:
    apiVersion: serving.knative.dev/v1
    kind: Service
    name: example
  service:
    port: 3000
  analysis:
    interval: 1m
    threshold: 10
    maxWeight: 50
    stepWeight: 5

[edude03]

(I haven't used flagger / knative in awhile but) I'm very excited to see this taking shape!

[arubino]

Hi @tombanksme! This looks great, are there any news on this PR?

[tombanksme]

I haven't heard anything back from the Flagger maintainers. I would be happy to finish up the PR if it's something that fluxcd would consider merging.

[aryan9600]

while i see the value added by this PR, is it not possible to use Gateway API as a bridge to get Flagger and Knative to work together: https://github.com/knative-extensions/net-gateway-api