Align API types between Porch and kpt

[liamfallon]

This PR aligns the kpt api types in Porch and kpt.

The only difference in the types is that the "copy-merge" update strategy in Porch does not exist in kpt. This PR adds that strategy to kpt. The copy-merge strategy uses a new RemoveStaleItems utility function, which is included in this PR as well.

Whne this PR is merged, the kpt API types can be used directly in Porch, allowing the cloned types in Porch to be deleted. Further, this will allow duplicate code in Porch to be removed bit by bit.

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[efiacor]

prob need a docs update to go with this right?
https://kpt.dev/reference/cli/pkg/update/
Also, did we switch to calling it upgrade in Porch?

[liamfallon]

prob need a docs update to go with this right? https://kpt.dev/reference/cli/pkg/update/ Also, did we switch to calling it upgrade in Porch?

When we bring in the "upgrade" we'll have to completely revisit the documentation in kpt. Today kpt is still on "update" but in porch we have replaced that with "upgrade", so when we make that port, we should update the docs. We may also port some of the documentation done on Porch to kpt for that area.