Add support for partialSuccess flag in entries.write request for oversized log entries

[losalex]

Is your feature request related to a problem? Please describe.
Today some requests might contain oversized log entries which cause entire write request to be dropped instead just of oversized log entries to be dropped (see log-limits for more information).
Describe the solution you'd like
The solution for this problem is to add partialSuccess flag as described in request-body.
Describe alternatives you've considered
Custom logic to truncate oversized entries could be implemented, but unfortunately truncating JSON structured log entries is complex and building generic solution for reduction JSON data is out of scope.