Introduced JWT based authentication for communication with registry

[johannesduesing]

Reason for this PR
The instance registry will require authentication based on JSON Web Tokens (JWTs) soon (as described here). The components of the Delphi system have to be able to generate valid JWTs based on a secret that is either passed in an environment variable or in the configuration file. They have to add the encoded token in the Authorization header of every HTTP request issued to the registry. For more information see the respective issue.

Changes in this PR

• The WebApi is now able to generate valid JWTs based on a secret specified in an environment variable (using HS256 signature algorithm)
• The WebApi sends an encoded valid JWT with every request issued to the registry
• A bug has been fixed that prevented the application from shutting down when trying to report shutdown to the registry

[codecov-io]

Codecov Report

Merging #41 into develop will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           develop     #41      +/-   ##
==========================================
- Coverage      1.4%   1.39%   -0.01%     
==========================================
  Files           18      19       +1     
  Lines          427     429       +2     
  Branches        31      33       +2     
==========================================
  Hits             6       6              
- Misses         421     423       +2

Impacted Files	Coverage Δ
...ain/scala/de/upb/cs/swt/delphi/webapi/Server.scala	0% <0%> (ø)	⬆️
...la/de/upb/cs/swt/delphi/webapi/Configuration.scala	0% <0%> (ø)	⬆️
...swt/delphi/webapi/authorization/AuthProvider.scala	0% <0%> (ø)
...t/delphi/instancemanagement/InstanceRegistry.scala	0% <0%> (ø)	⬆️

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