flux export can be improved with managedFields

[stealthybox]

Currently, flux export seems to write out every field. It doesn't even omit defaults.
It looks possible to only print the fields that were specified by kubectl.

This should probably be an optional feature because of the danger of missing things.
The option could take a comma-separated list of field owners to print. There could be a reasonable default.
This would allow us to have configurable field owners in flux-cli, kustomize-controller, or helm-controller in the future.

[stealthybox]

Maybe flux create ... commands could also print a message encouraging you to add the YAML to your repo?
There could be an -o=yaml or an --output-file flag for scripting/automation.

It's a bit awkward to throw away the client-generated object when the object exported from the cluster has many extra fields that don't fully capture the intent necessarily.