flux controller

**Describe the problem/challenge you have**

kapp-controller and its ytt templating is great, and allowed me to overcome some limitations in kustomize flux workflows (see https://github.com/kubernetes-sigs/kustomize/issues/4636)

However, kapp-controller and ytt does not integrate yet well with flux ecosystem, for flux consumer projects :
* the source (git repo/oci) are fetched independently :
* from kapp fetch section
* from flux source controller https://fluxcd.io/flux/components/source/
* with different feature sets, e.g. kapp-controller does not yet support [event-based pulls](https://kubernetes.slack.com/archives/CH8KCCKA5/p1674134387444429)
* kapp-controller does not yet support notifications that flux could offer through the notification controller https://fluxcd.io/flux/components/notification/(see https://github.com/vmware-tanzu/carvel-kapp-controller/issues/522)
* flux users also have to learn the kapp UX, and adapt their tooling see https://github.com/derailed/k9s/pull/1830
* kapp controller does not yet have AFAIK builtin monitoring that flux could provide, see https://fluxcd.io/flux/guides/monitoring and https://github.com/vmware-tanzu/carvel-kapp-controller/issues/789

**Describe the solution you'd like**

Carvel tools to also be integrated as a flux extension, see https://fluxcd.io/ecosystem/#flux-extensions

* step 1: a "all-in-one" carvel flux controller:
* leveraging flux source events
* with a consistent CRD ux and feature set to [kustomize controller](https://fluxcd.io/flux/components/kustomize/) such as
* [spec.suspend to true](https://fluxcd.io/flux/components/kustomize/kustomization/#reconciliation)
* enabling all of carvel templating (e.g. ytt,cue...) and deploy (in particular kapp)
* step 2: fine grain controllers
* a carvel templating flux controller
* an option to leverage kapp for both
* existing [Kustomize controller](https://fluxcd.io/flux/components/kustomize/)
* carvel templating flux controller

**Anything else you would like to add:**

This idea has been discussed in the carvel project history, and further exchanged during the community meeting on Jan 11th 2023, see https://hackmd.io/G8dN30WvQl-8Sirnp8AgRA?view#January-11-2023-Agenda

[Additional information that will assist in solving the issue.]

The https://fluxcd.io/flux/gitops-toolkit/source-watcher/ seems the entry point to spiking new flux controllers

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