Camunda 8: add integration-tests for gRPC Job-Workers

[bruderj15]

• Test Job-Streaming
• Test configured gRPC
• Test default usage of REST over gRPC

[bruderj15]

This currently cannot be done natively due to camunda/camunda#45177.

If my suggestion there doesn't work out, consider some reflection (okay for tests).

[bruderj15]

This currently cannot be done natively due to camunda/camunda#45177.

Will be possible with 8.9.1