Allow Plugins to Extend Known Elements

[LeonBein]

As a simulation user, when providing a simulation configuration with plugin-handled elements, e.g. <catchEvent>.

Expected Behavior I expect there to be no warning if the element is supported by a plugin e.g., the EventArrivalRatePlugin.

Actual Behavior Instead, there is a warning about Element XXX of simulation scenario not supported (see https://github.com/bptlab/scylla/blob/7d1d3039b9cdce6cb6bf35c346830deb4a64b0c1/src/main/java/de/hpi/bpt/scylla/parser/SimulationConfigurationParser.java#L252C63-L252C99).

Proposed solution: Parser plugins should be allowed to mark elements as known so this defect does not occur anymore.