Python wrapper should support Comparer builder

[grecinto]

The Comparer that was created for JSONDB is CEL expression based. Default will be to support CEL expression string to be passed in from Python by the developer.

However, on "one of next iterations", we should support CEL expression comparer builder to allow simple definition of Comparer of keys without forcing to learn CEL expression syntax.

[grecinto]

IndexSpecification struct does this.