Previously, when the timezone was missing from the datetime string
and the pattern, UTC was used, instead of the session defined timezone.
Moreover, if a timezone was included in the datetime string and the
pattern then this timezone was used. To have a consistent behaviour
the resulting datetime will always be converted to the session defined
timezone, e.g.:
```
SELECT DATETIME_PARSE('2020-05-04 10:20:30.123 +02:00', 'HH:mm:ss dd/MM/uuuu VV') AS datetime;
```
with `time_zone` set to `-03:00` will result in
```
2020-05-04T05:20:40.123-03:00
```
Follows: #54960
(cherry picked from commit 8810ed03a209cc8fe1bad309a81e85b56a39da27)
363e994171