Allow providers to overwrite structural type scalar property order

### What problem are you trying to solve?

Structural type scalar property order is determined internally by Core using `PropertyNameComparer`, and can't be overwritten by providers. However a different order might be warrented in some cases. For example: JSON entities that support inheritance hierarchies might want to put the discriminator as the first property for faster materialization. This is currently the case for the Cosmos DB provider.

### Describe the solution you'd like

Some way to overwrite scalar property order for structural type instances

Start by tracing Core's PropertyNameComparer and the structural type scalar property ordering, then compare how the Cosmos DB provider currently handles discriminator placement. Done means providers have a defined way to override scalar property order for structural type instances without breaking existing ordering behavior.

Written by the indexing model from the issue text.