Refactor HTTP message parsing to build on top of new PSR-7 implementation

[clue]

This changeset refactors the HTTP message parsing logic in the internal ClientRequestStream and RequestHeaderParser classes to build on top of a new PSR-7 implementation. This brings us one step closer to eventually replace the dated RingCentral implementation (#331) and eventually support PSR-7 v2 (#513). Most notably, this means the Browser will now return instances of our Response class (#518) instead of using the legacy RingCentral classes.

Other than that, this is a purely internal change that comes with 100% code coverage and does not otherwise affect the public API, so it should be safe to apply. I've tried to keep changes to a minimum to ease review, but there's definitely some room for improvement in follow-up PRs.

This builds on top of the recent changes for the Response and ServerRequest classes (#518 and #519), but this time doesn't show a noticeable impact on performance during my benchmarks. As a consequence, I consider this mostly an internal refactoring only.

Once merged, I'll file follow-up PRs to refactor the Uri class accordingly. If you enjoy this change and want to help us continue to ship more improvements, consider supporting this project, for example by becoming a sponsor ❤️

Builds on top of #519, #518, #480, #432, #370, #170 and others

[SimonFrings]

Went through all changes, looks good so let's get this shipped 👍

[WyriHaximus]