Adapt Crawler to latest changes of the registry interface

[johannesduesing]

Reason for this PR
The interface of the instance registry has been changed lately. The reason for this was to better support the requirements of the Delphi-Management state management. The relevant change is:

• The attributes linksTo: List[InstanceLink] and linksFrom: List[InstanceLink] have been added to the Instance class

This PR adapts the Crawler to this change. The same has been done for the WebApp and WebApi.

Changes

• Added file InstanceLink.scala containing the JSON support
• Added new attributes to Instance

[codecov-io]

Codecov Report

Merging #33 into develop will decrease coverage by 0.09%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           develop     #33     +/-   ##
=========================================
- Coverage     8.22%   8.12%   -0.1%     
=========================================
  Files           43      44      +1     
  Lines         1155    1169     +14     
  Branches        34      27      -7     
=========================================
  Hits            95      95             
- Misses        1060    1074     +14

Impacted Files	Coverage Δ
...a/de/upb/cs/swt/delphi/crawler/Configuration.scala	0% <ø> (ø)	⬆️
...lphi/crawler/instancemanagement/InstanceLink.scala	0% <0%> (ø)
.../crawler/instancemanagement/InstanceRegistry.scala	0% <0%> (ø)	⬆️
...t/delphi/crawler/instancemanagement/Instance.scala	0% <0%> (ø)	⬆️

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