Composition implementation prototype

[KnVerey]

This is a fairly complete implementation of kubernetes/enhancements#2299. This PR is way too big to review; I'll open separate, more granular PRs that reference this one.

Besides the TODOs in this PR, work remaining to make Composition usable includes:

• We need a generator that is equivalent to the resources field and invokable from the extensions fields. The ResourceGenerator generator in this PR only handles files, not directories, and therefore not bases and has no tests. Ideally the new generator should be an extraction of the code driving the field, and the field should switch to using it. Package structure is a big challenge here, since the generator will need to be able to expand bases, which uses krusty, which is in the api modules, which imports plugins. I.e. it probably can't be built the same way we've built all the builtin generators historically. Convert resources and components to be backed by a reusable generator #4402
• Generators built as Go plugins don't work in the transformers field for implementation reasons I want to try changing. Enable built-in generators to be used in the transformers field #4403
• When you use a builtin from the transformers field, the fieldSpecs field defaults to empty instead of the usual defaults. That's a pretty big usability issue for Composition. Can we change that (are transformers with empty fieldSpecs conceptually valid?) or introduce a way of building on instead of overriding the defaults? Enable explicitly invoked transformers to use default fieldSpecs #4404

fyi @jeremyrickard @natasha41575

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