Add support for psr/http-message v2

[SimonFrings]

I recommend adding support for psr/http-message v2, which was released in April and requires PHP 7.2+. To ensure compatibility with psr/http-message v2 across all currently supported versions in http (PHP 5.3+), we will need to implement some additional workarounds for older PHP versions. These workarounds are necessary not only for the current v1 of http, but also for the upcoming v3, which will support PHP 7.1+. We will also need to include additional tests to ensure our compatibility with psr/http-message v2.

Additionally we need to remove the dependency for ringcentral/psr7 as discussed in #485, which currently has a dependency on psr/http-message v1. This project has not received updates since 2020, and it is unlikely to receive support for psr/http-message v2 anytime soon. To ensure compatibility with psr/http-message v2, we must first remove ringcentral/psr7.

I'll update this ticket as we make progress 👍

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