fix: duplicated objects when charts are nested

[Chriscbr]

Fixes #31

I think the expected behavior from users is that in most circumstances, you should be able to directly apply resources generated by cdk8s synth to your k8s cluster (e.g. via kubectl apply -f dist/). However, it's possible for nested charts to cause resources to be listed twice. This PR resolves this by de-duplicating resources so that each one is only rendered once during a given "synth" call.

This PR makes the assumption that nesting charts within each other is implicitly a feature (I don't believe there are any other ways to arrive at charts containing the same resource twice?). It's possible we could instead add some kind of validation to prevent charts from being nested to resolve this issue instead, but this would become problematic if users are defining constructs that include charts.

Questions:

• Should there be some way to synthesize a specific chart within an app from the CLI?
• Should there be an option for synthesis (e.g. a CLI flag) to override this and allow duplicate objects?

[Chriscbr]

Per our discussion, I updated all synth methods so that the deduplication behavior is consistent across all of them.