expr: support casting varchar -> timestamptz with timezone hour but not minute

[xiangjinwu]

Example:

2023-09-27 15:40:14.295357+08

This is a mixture of ISO 8601 and RFC 3339, but is not fully compliant with neither and unsupported by chrono (unless a custom format is used) or speedate. However, it is the default output format of PostgreSQL.

• 2023-09-27T15:40:14.295357+08: ISO 8601, with date/time separator as T rather than a space.
• 2023-09-27 15:40:14.295357+08:00: RFC 3339, with timezone minutes.