[hd, usdImaging] Rework material binding schemata and resolution.

[hd, usdImaging] Rework material binding schemata and resolution.

hd:
- Remove 'bindingStrength' from HdMaterialSchema. This is an authoring affordance that doesn't need to be transported to a render delegate.
- Update HD_API_VERSION.

usdImaging:
- Add schemata to represent direct and collection-based material bindings.
- Update materialBindingAPIAdapter to build and invalidate data sources based on the schemata.
- Move direct binding flattening from hd to usdImaging.
- Add a scene index to compute the resolved material bindings. Currently, it just wraps the flattened direct binding for a given purpose; this will be updated to factor in collection material bindings in a future change.

(Internal change: 2310706)