use kapp as a Go Module

**Describe the problem/challenge you have**

We'd be very excited if we would not have to shell out to Kapp from an orchestration tool but use as a Go module, which would dramatically improve the error handling and resilience.

**Describe the solution you'd like**

Similar to https://github.com/vmware-tanzu/carvel-ytt/blob/develop/examples/integrating-with-ytt/apis.md#as-a-go-module there could be general commitment to an exported pkg or API, that is subject to change as long as Kapp version is < 1

**Anything else you would like to add:**

Issue created on positive initial feedback from https://kubernetes.slack.com/archives/CH8KCCKA5/p1659086135812959

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