Implement parse_datetime() function

Implement parse_datetime() function

This commit adds parse_datetime() function, which implements datetime
parsing with extended features demanded by upcoming jsonpath .datetime()
method:

 * Dynamic type identification based on template string,
 * Support for standard-conforming 'strict' mode,
 * Timezone offset is returned as separate value.

Extracted from original patch by Nikita Glukhov, Teodor Sigaev, Oleg Bartunov.
Revised by me.

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
Reviewed-by: Anastasia Lubennikova, Peter Eisentraut