Add ET_Filter to create more complex queries

[janua]

Problem

When currently using ET_Get it is impossible to put a Complex filter inside a Complex filter. This makes writing actual complex queries useless as they won't work.

For example, I can't query all ListIds I want in one query by saying 123 or 456 or 789 or 999.

Rough example query

And(And(Or(A, B) C), D) which equates to D and (C and (B or A))

Fix

This adds ET_Filter which allows for more complex nested queries.

Now we can pass complex nested queries to ET_Get.