Enable partial evaluation from the java API

[MFAshby]

what:
data filtering is a very powerful capability of oso. It allows me to apply attribute based access control to both read and search operations using write the same, simple policy for both. I'd like to make that capability available on Java also.

why:
I haven't yet found another authorization library with this capability in the java ecosystem!

how:
Implement the necessary types and conversion routines to be able to evaluate policies with Variables in them and return Expressions.