`kctrl`: When adding a package repo, include option to create the namespace if not existing

[ThomasVitale]

Describe the problem/challenge you have
When I add a Carvel package repository to a cluster, I first need to create a dedicated namespace explicitly.

1. kubectl create namespace kadras-packages
2. kctrl package repository add -r kadras-repo --url ghcr.io/arktonix/kadras-packages:0.5.1 -n kadras-packages

Describe the solution you'd like
I'd like to skip the explicit creation of the namespace and have it created as part of the kctrl package command, similar to how the helm CLI works. For example, the functionality could be enabled via a --create-namespace flag.

kctrl package repository add -r kadras-repo --url ghcr.io/arktonix/kadras-packages:0.5.1 -n kadras-packages --create-namespace

Anything else you would like to add:
One of the reasons for this feature request is to make the onboarding experience even simpler when introducing people to Carvel packaging or when using a Carvel metapackage as the foundation of projects/tutorials relying on a Carvel-based platform.

With this feature, that would result in three operations rather than four:

1. deploy kapp-controller
2. add the package repository
3. install the package

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[praveenrewar]

I agree that this is definitely useful in terms of reducing the number of commands that need to be run to add repositories or install packages but I remember that we have had some similar discussion but I can't seem to recall the details.
cc @100mik @cppforlife

[100mik]

I think it is thoughtful that we are doing this only for PackageRepositories. As generally they would reside any installs will reside in the same namespace that is already created.

There is a case for doing this for installs as well since a PackageRepository might be added to the global namespace. But I think a good first step would be to do this only for repository adds as it is the more common use case.

[ThomasVitale]

@100mik as agreed, I'm starting working on this. Should we go with the --create-namespace flag or would you prefer a different naming strategy?