Support `schemaUri` property of EXT_structural_metadata

[azrogers]

As noted in #727, though EXT_structural_metadata can specify a schemaUri that contains the URI of a JSON file to load instead of the embedded schema, we don't currently support this property. This change adds that support.

As part of the change, I gave the generate-classes tool some idea of what a "shared asset" is, so it can now generate a shared asset property directly rather than needing to mess with derived classes like in the case of Image/ImageSpec. The largest change, by lines of code, is that a getSizeBytes method is now generated for every type. This is necessary for the SharedAssetDepot to know when it's "full."

Code needs cleanup, especially the JavaScript code which I don't have a formatter set up for. Also need to test in each of the runtimes - if there's any changes needed, they'll be small, but I haven't checked yet.