Allow datetime values in JsonbValue

Allow datetime values in JsonbValue

SQL/JSON standard allows manipulation with datetime values.  So, it appears to
be convinient to allow datetime values to be represented in JsonbValue struct.
These datetime values are allowed for temporary representation only.  During
serialization datetime values are converted into strings.

SQL/JSON requires writing timestamps with timezone in the same timezone offset
as they were parsed.  This is why we allow storage of timezone offset in
JsonbValue struct.  For the same reason timezone offset argument is added to
JsonEncodeDateTime() function.

Extracted from original patch by Nikita Glukhov, Teodor Sigaev, Oleg Bartunov.
Revised by me.  Comments were adjusted by Liudmila Mantrova.

Discussion: https://postgr.es/m/fcc6fc6a-b497-f39a-923d-aa34d0c588e8%402ndQuadrant.com
Discussion: https://postgr.es/m/CAPpHfdsZgYEra_PeCLGNoXOWYx6iU-S3wF8aX0ObQUcZU%2B4XTw%40mail.gmail.com
Author: Nikita Glukhov, Teodor Sigaev, Oleg Bartunov, Alexander Korotkov, Liudmila Mantrova
Reviewed-by: Anastasia Lubennikova, Peter Eisentraut