news: add news file for #4319

Signed-off-by: Balazs Scheidler <bazsi77@gmail.com>

news/feature-4319.md

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+`date-parser()`: add `value()` parameter to instruct `date-parser()` to store
+the resulting timestamp in a name-value pair, instead of changing the
+timestamp value of the LogMessage.
+
+`datetime` type representation: typed values in syslog-ng are represented as
+strings when stored as a part of a log message.  syslog-ng simply remembers
+the type it was stored as.  Whenever the value is used as a specific type in
+a type-aware context where we need the value of the specific type, an
+automatic string parsing takes place.  This parsing happens for instance
+whenever syslog-ng stores a datetime value in MongoDB or when
+`$(format-date)` template function takes a name-value pair as parameter.
+The datetime() type has stored its value as the number of milliseconds since
+the epoch (1970-01-01 00:00:00 GMT).  This has now been enhanced by making
+it possible to store timestamps up to nanosecond resolutions along with an
+optional timezone offset.
+
+`$(format-date)`: when applied to name-value pairs with the `datetime` type,
+use the timezone offset if one is available.