The JSONB jbvDatetime type has a field for offset, and displays the time
in that offset. For example, when the time zone GUC is set to
America/New_York, the jsonpath `timestamp_tz()` method returns a value
that displays a parsed value with its offset, not the local offset:

    david=# set time zone 'America/New_York';
    SET
    david=# select jsonb_path_query_tz('"2024-08-15 12:34:56+10"', '$.timestamp_tz()');
        jsonb_path_query_tz
    -----------------------------
    "2024-08-15T12:34:56+10:00"

This was not true for values parsed by `timestamp_tz()` that lacked an
offset. It correctly assumes the local time zone, but displays it in
UTC:

    david=# select jsonb_path_query_tz('"2024-08-15 12:34:56"', '$.timestamp_tz()');
        jsonb_path_query_tz
    -----------------------------
    "2024-08-15T16:34:56+00:00"

To fix this inconsistent behavior, determine the offset for values being
cast from `DATEOID` and `TIMESTAMPOID` types to `jpiTimestampTz` and store
it in the resulting jbvDatetime value. With this change, the result now
preserves the offset just as it does when converting from offset-aware
values:

    david=# select jsonb_path_query_tz('"2024-08-15 12:34:56"', '$.timestamp_tz()');
        jsonb_path_query_tz
    -----------------------------
    "2023-08-15T12:34:56-04:00"

Author: David Wheeler
Reviewed-by: Junwang Zhao and Jeevan Chalke
Discussion: https://postgr.es/m/7DE080CE-6D8C-4794-9BD1-7D9699172FAB%40justatheory.com