Add subresults support for reportportal report plugin

[seberm]

This PR adds support for tmt subresults for tmt reportportal plugin. It tries to map the subresults as child items of parent test result in a specific launch. There are also small code improvements like the possibility to ignore the SSL verification, or the usage of Python properties instead of class methods.

Please don't review this PR until it is marked as a draft.

More info:

• https://reportportal.io/docs/api/service-api/start-child-item-using-post-1

TODOs:

• Find a way to map the subresults to parent test/step reportportal launch items
• Make the subresults import optional via argument? What should be the default behavior?
• Try to resolve status:interrupted issues for subresults in RP, why is this happening?

Related:

• Add support for test phases #2826
• https://issues.redhat.com/browse/TT-306

Pull Request Checklist

• implement the feature
• write the documentation
• extend the test coverage
• update the specification
• adjust plugin docstring
• modify the json schema
• mention the version
• include a release note