Allow kapp deploy go code to be used with an fs.FS

[ncdc]

Describe the problem/challenge you have
I am doing a POC where I want to call app.NewDeployCmd() and then execute it programatically. I also want to pass it an fs.FS implementation because the content I want to apply to my cluster is coming from an in-memory file system, not from the OS.

Describe the solution you'd like
The ability to pass an fs.FS when invoking deploy programmatically.

Anything else you would like to add:
No

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