Add support for multiple Marker instances on a single logging event (SLF4J 2.0.0)

[brenuart]

Since SLF4J 2.0.0, the SLF4J logging API assumes the possibility of multiple Marker instances in a logging event. Consequently, ILoggingEvent#getMarker() has been deprecated in favour of ILoggingEvent#getMarkerList() which now returns a list of Marker instances.

Although the SLF4J Logger interface does not allow for more than one Marker instance per events, the LoggingEventBuilder allows to add several.

The LLE LogstashMarkersJsonProvider and TagsJsonProvider must be adapted to take this into account.