Split 'Query.Language.ItemType' in two sub-types

The QR_VIA_OP/QR_VIA_LUXI types in Python are using yet another
validation mode: QR_VIA_OP is the base type, and QR_VIA_LUXI extends
it (when doing luxi queries). But on the wire they have the same
representation.

To accommodate this properly, we split the ItemType in two: a
QueryTypeOp and a QueryTypeLuxi, joining them back together in
ItemType itself. This requires custom serialisation/deserialisation,
but allows us to express correctly that at opcode level, we only
accept a QueryTypeOp, but at Luxi query level, we accept either of
them.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>