This will be used in clients to build the filters for query2.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

This parser reads only the format described by the query2
design document: either an empty filter or an OR operator
with equality checks as operands.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>