Add coverage portlet for pull-request-monitoring plugin

[simonsymhoven]

Implement portlet for the https://github.com/jenkinsci/pull-request-monitoring-plugin. The portlet visualize the aggregated results for line, method and instruction coverage of the CoverageResult for the CoverageAction. I used the existing diagram of the CoverageResult as a guideline and implemented the following portlet based on the existing API for the different cases:

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[simonsymhoven]

Due to #182 and failing checks because of Jenkins baseline version (see #196), marked this pr as draft again. Will come back later after finished #182 and extended the change-request coverage api to implement better portlet for pull-request-monitoring plugin.

[simonsymhoven]

I started to wrap the interfaces into abstract classes. Will come back here and update the pr after jenkinsci/pull-request-monitoring-plugin#75 finished and released.