Introduce hacky support for kind composition

[sdboyer]

This PR introduces (hacky! early!) support for composition of Composable kinds into Resource kinds.

The eventual goal is that we can describe kind composition declaratively in enough detail that all the work of doing composed validation can be handled generically here in the base kind implementations provided by kindsys. Doing so is prerequisite to doing full, composed validation on the apiserver, rather than having to rely webhooks (which would get pretty noisy).

For now, though, the implementations have to be hardcoded per-kind. I'm pulling up a corresponding PR in Grafana that does that for dashboards.

[sdboyer]

Need to fix up tests, but this is working as-is for composed dashboard validation over in grafana/grafana

[sdboyer]

CI passing now.

We don't have a direct way of testing this in kindsys right now, but we will once we have a corpus. For now, best thing is just knowing that it works in core, for dashboards (see grafana/grafana#72934)

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