Support non-autoloaded vector properties - model and change tracking

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See parent #37277 for the general feature design.

• On the model side, Common/unified EF experience for vector search #36350 includes adding core metadata support for vector properties, so this issue ultimately depends on that.
• However, we should be able to introduce a more general "partially-loaded-by-default property" distinction into the model; all vector properties would be partially-loaded-by-default (by convention?), but the mechanism would (at least in theory) be usable also for other property kinds. This can happen without depending on Common/unified EF experience for vector search #36350.
• Once the modeling support is there, change tracking should be updated to support the new partially-loaded property concept.
  • Note: it should be possible for users to set a new value to the vector property and have that persisted (that would need to happen if the other source properties on the entity change).