Refactoring: Create passage-detector-handler module with PassageDetectorHandler lambda

[joelbinn]

The PassageDetectorHandler shall handle all operations for passage detection:

• start gate passage (today: start-gate-passage-detected-lambda)
• plit gate passage (today: split-gate-passage-detected-lambda)
• finish gate passage (today: spli-gate-passage-detected-lambda)
• Do this by merging backend/start-gate-passage-detected-lambda, backend/split-gate-passage-detected-lambda and backend/finish-gate-passage-detected-lambda into one module and one handler (change name of maven module, handler and package).
• Update SAM-template (see table with REST-routes för more info).