Implement UserDefinedValueCondition

One way of supporting external triggers (e.g. from Test automation tooling) is to use the UserDefinedValueCondition in scenarios (see note at the end of the section): https://www.asam.net/index.php?eID=dumpFile&t=f&f=4908&token=ae9d9b44ab9257e817072a653b5d5e98ee0babf8#_variables

The values can then be set from external side and be checked by the scenario engine. With this issue two things will be implemented:

1. An interface to the engine to set user defined values
2. The UserDefinedValueCondition, which checks the values