Implement Session PUT method

[rebel-l]

The /session endpoint has a PUT method to create or update the sessions data. The documentation you can find on Swagger hub.

If the id in the payload is omitted a new session should be started which means a new id must be created.

Note: There are some dependencies to other issues which are used here.

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[rebel-l]

open:

• reorganize code to get more readable
• write unittests for endpoint

[rebel-l]

Everything implemented and merged