Merge Timer Event and Event Arrival Rate Plugins

[LeonBein]

Both plugins handle delays due to events and could be combined.
Currently, the following differences and deficiencies have been analyzed:

• EAR plugin only supports intermediate events, while TE plugin supports all kinds of bpmn events (even end events, which is potentially unnecessary)
• EAR plugin uses time distributions, while TE plugin uses ISO 8601 standard fixed time intervals (see also #1 )
• None of the two support date based or cyclic triggers (as examplified at https://docs.camunda.org/manual/7.19/reference/bpmn20/events/timer-events/)
• For both, writing tests in general is necessary
• Specifically (manually) testing whether boundary events properly work is necessary
• More to be determined

[LeonBein]

This issue was created based on a task note made by @bdaase in https://github.com/bptlab/scylla/projects/13.
Here is the original note:

Timer Events Semantic

With having implemented a way to give incoming message events a time after they are triggered, this should be possible to reuse (see card: Time of receiving message events).

Another point that has to be tackled is to give timer events the right semantics which is at least to adapt the notation of the timer events to the notation of all other events. That includes to bring all boundary events trigger time distributions together in one source file (the .sim file) using the .xml syntax and having the same configuration options as the other boundary events. Furthermore the time and date format in the .bpmn file (using a different notation according to the ISO 8601 standard) should also support instance specific evaluated expressions (example at https://docs.camunda.org/manual/7.6/reference/bpmn20/events/timer-events).