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