Serialise and deserialise the new rating field of message. This is
backward-compatible and does not affect function arguments, differently from
earlier iterations of the ENGINE-959 change set.
The new field has an enum type and is therefore serialised as a JSON non-scalar,
as per the conventions in README.ml .
Thanks to Jörg Knobloch who found the issue starting from pEp for Thunderbird.
This follows the engine change ENGINE-959, after decrypt_message has been
reverted to its previous API (rating output parameter re-added). The original
change set designed for the incompatible engine API is not in this branch.