Perform charset-aware decoding of request bodies

[JonasKunz]

What does this PR do?

Respects the charset from the Content-Type header when capturing HTTP Client request bodies. Note that it might be possible that the bytes are still not decodeable (e.g. because the content is actually compressed). In that case we fallback to simply casting the bytes to chars.

Checklist

• [x] This is an enhancement of existing features, or a new feature in existing plugins
  • [ ] ~I have updated CHANGELOG.asciidoc~ Don't think we need a changelog because this feature is not publicly visible yet
  • [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~
• [ ] This is a bugfix
  • [ ] I have updated CHANGELOG.asciidoc
  • [ ] I have added tests that would fail without this fix
• [ ] This is a new plugin
  • [ ] I have updated CHANGELOG.asciidoc
  • [ ] My code follows the style guidelines of this project
  • [ ] I have made corresponding changes to the documentation
  • [ ] I have added tests that prove my fix is effective or that my feature works
  • [ ] New and existing unit tests pass locally with my changes
  • [ ] I have updated supported-technologies.asciidoc
  • [ ] Added an API method or config option? Document in which version this will be introduced
  • [ ] Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• [ ] This is something else
  • [ ] I have updated CHANGELOG.asciidoc