Add query function isdefined(PropertyPath) to determine whether an instance has a property of that name

We allow clients to add dynamic/undeclared properties to instances of open types, but we can't express filter conditions on the existence of a property of that name.

This is also useful in the context of data aggregation to determine if a property has been "aggregated away". Here the extension will define that defined(...) returns false even for declared properties if they are not defined on an entity of the result set as an effect of groupby(), rollup(), or aggregate().