gdatetime: Add g_date_time_format_iso8601() convenience function

This is a simple wrapper around g_date_time_format_iso8601() which
always produces ISO 8601 dates, without people having to remember the
format string for them (and with the convenience of terminating UTC
dates with ‘Z’ rather than ‘+00’).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1438

docs/reference/glib/glib-sections.txt

@@ -1983,6 +1983,7 @@ g_date_time_to_utc
 
 <SUBSECTION>
 g_date_time_format
+g_date_time_format_iso8601
 </SECTION>
 
 <SECTION>