Capture request body for HttpUrlConnection

[JonasKunz]

What does this PR do?

Adds support for capturing request bodies for HttpUrlConnection, also cleans up some shared code and adds test cases.

Not adding changelog and docs yet because this feature is hidden behind an internal feature flag.

Checklist

• [x] This is an enhancement of existing features, or a new feature in existing plugins
  • [ ] ~I have updated CHANGELOG.asciidoc~
  • [x] I have added tests that prove my fix is effective or that my feature works
  • [ ] ~Added an API method or config option? Document in which version this will be introduced~
  • [ ] ~I have made corresponding changes to the documentation~