[1.x] Support PSR-7 v2 along side v1

[WyriHaximus]

This changeset introduces our PSR-7 v2 implementation alongside our PSR-7 v1 implementation and goes with v1 or v2 depending on which psr/http-message version is installed. This is admittedly not my best code, nor am I proud of it, but it gets the job done.

This relies on clue/reactphp-http-proxy#65 being merged first.

Implements #513 for react/http v1.

[cyrnetix]

Great work, would be nice of have this be merged? I ran into an issue while updating some vendors recently who also bumped their version to v2 already.

[WyriHaximus]

Hey @cyrnetix, currently waiting for @clue's review. Something you could help with here is to try out this branch in the project you got stuck with and report back here how that works out for you. As this is an absolute unit of a PR.

[cyrnetix]

My colleague tested most of the flows as much as possible.

This were his findings:

Four our case there was another dependency issue involving clue/http-proxy-react, which depends on ringcentral/psr7. The ringcentral/psr7 vendor requires psr/http-message: ~1.0. To address this, there is an open PR (#65) in the clue/http-proxy-react repository that removes this dependency.

So, along with the PR #549, I continued the testing with also switching the clue/http-proxy-react vendor to that branch. After upgrading a few other vendor packages, everything now seems to be working correctly.

[WyriHaximus]

My colleague tested most of the flows as much as possible.

This were his findings:

Four our case there was another dependency issue involving clue/http-proxy-react, which depends on ringcentral/psr7. The ringcentral/psr7 vendor requires psr/http-message: ~1.0. To address this, there is an open PR (#65) in the clue/http-proxy-react repository that removes this dependency.
So, along with the PR #549, I continued the testing with also switching the clue/http-proxy-react vendor to that branch. After upgrading a few other vendor packages, everything now seems to be working correctly.

Great! Had a call yesterday with @clue about this PR, among things. Been working on it to reduce the changed line count from +4.6K/-2.1K to +2.3K/-9 so it's easier to review. Also updated clue/http-proxy-react to yesterday's release that addresses the ringcentral/psr7 concern.