usd: add a UsdSchemaRegistry function for identifying schematics layers

[mattyjams]

Hello!

Following https://github.com/PixarAnimationStudios/USD/commit/a32872440a542d5ef16b7be610a7a7f5a10f0294 that landed in v23.05, schematics layers read by the schema registry are no longer aggregated into a single registry.usda layer as they were in v23.02 and earlier. This has made it a bit more cumbersome for unit testing and other layer lifetime debugging as there isn't really a clean way to differentiate these now anonymous schematics layers from other layers that contain user content when using SdfLayer::GetLoadedLayers(), for example.

I added a IsSchematicsLayer() function to UsdSchemaRegistry that can be used to make that distinction and help filter out any of these schematics layers.

• [X] I have verified that all unit tests pass with the proposed changes
• [X] I have submitted a signed Contributor License Agreement