Support stdin kubeconfig for add command

[bcdurden]

Is your feature request related to a problem? Please describe.
Potentially, but likely not. Immutable workstations may have issue downloading files, something like this could also be used to generate a dynamic kubeconfig to be consumed by another tool

Describe the solution you'd like
Using MacOS terminal as an example, I'd like to use pbpaste or similar utility (echo) to feed a kubeconfig to kubecm add from stdin.

Describe alternatives you've considered
pbpaste can be used to output to a file that can then be read into kubecm add using the previous example. However, this doesn't flow as well and other K8S utilities make use of piping natively which would enable this utility to become part of that flow. See Carvel's toolsuite for an example (heavily used in VMware's TKG products as a replacement for helm with many extra features).

Additional context
For Rancher products, this would make for a very useful utility in copy-pasting kubeconfigs from the Rancher Server UI without needing to download the file itself

[sunny0826]

Thanks for your feedback, I will implement this feature

[sunny0826]

@bcdurden HI, Maybe this PR #282 does what you want it to do.

I read the kapp description and what you want is something like kapp -y deploy -a my-app -f <(kustomize build . /my-app)?