AssetSubset -> EntitySubset

[OwenKephart]

Summary & Motivation

This converts the AssetSubset class into a generic EntitySubset class. This allows us to create subsets of arbitrary types without needing to create new subclasses, while at the same time allowing us to indicate via the type system what kind of EntitySubsets are allowed for certain methods / properties. For example, AssetGraphSubset currently only supports EntitySubset[AssetKey] (although this will likely change in the future).

How I Tested These Changes

Changelog [New | Bug | Docs]

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