Add the ability to reference a GitRepository resource

Right now there is some overlap when using Flux and Kapp controller together. Flux already provides the source controller to be able to poll a git repo. Kapp also has this functionality. It would be really nice to be able to use the Flux controller's GitRepository resource as a source for the Kapp controller. Today I can use the `app.spec.fetch.http` to reference the tar that the GitRepository creates, but that is not sustainable. being able to reference the GitRepository natively within the app spec would make this really easy to use and essentially turn Kapp into a flux controller .

I would propose that we add `app.spec.fetch.GitRepository` (or something similar) to the spec so that this can be done natively.

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