This method is to read a particular index from valueBuffer for a property.
It was in EntityMaterializerSource as an instance method in order for provider to influence how values are read from valueBuffer.
In new query pipeline, providers are supposed to convert valueBuffer read to their database object read. Further, even if they want to create a ValueBuffer for read, the valueBuffer should have values after all custom processing.
Hence there is no particular need to make this overridable behavior. Making it extension method simplify quite a lot of things.

(note: functionality to use this was available anyway since the method it invoke was also publicly exposed as static.

…haper

Part of #18923
Part of #10140

…minatorCondition

If DiscriminatorCondition returns null for IEntityType then return null instance.

Part of #18923