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.

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?

Blocked by:

• Multiple reportportal plugin improvements and fixes #3356

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