pgsql: 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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/66c74f8b6e347ba5830bf06468bef8081601c187

Modified Files
--------------
src/backend/utils/adt/date.c | 11 +-
src/backend/utils/adt/formatting.c | 291 ++++++++++++++++++++++++++++++++++++-
src/include/utils/date.h | 3 +
src/include/utils/formatting.h | 3 +
4 files changed, 296 insertions(+), 12 deletions(-)