WIP: Add suport for Amphp PSR18 Client

[thePanz]

Note: the use Amp\Http\Client\Psr7\PsrHttpClient is still a WIP, and the support for PSR-18 has been added only on the v2 branch

Only on that branch the PsrHttpClient is implementing the ClientInterface

Note: The library only supports the psr/http-message:v1 contract

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Related tickets	fixes #240
Documentation	if this is a new feature, link to pull request in https://github.com/php-http/documentation that adds relevant documentation
License	MIT

What's in this PR?

Implement support for amphp/http-client-psr7

Some notes:

1. the amphp/http-client-psr7 main branch is NOT implementing the PSR ClientInterface
2. the v2.x branch is effectively implements the PSR ClientInterface, but relies on psr/http-message v1
3. looks like the PSR layer is not actively maintainer (master last commit: 3 years ago, v2.x last commit: 2 years ago)
4. there is a PR bringing the library up https://github.com/amphp/http-client-psr7/pull/10 by @danog but not merged

Checklist

• [ ] Updated CHANGELOG.md to describe BC breaks / deprecations | new feature | bugfix
• [ ] Documentation pull request created (if not simply a bugfix)

To Do

• [ ] Make ci test the installation of amphp
• [ ] Clarify the min version required of the amphp/http-client-psr7 library
• [ ] Update Composer/Plugin.php according to the documentation, by adding amphp/http-client-psr7

[nicolas-grekas]

Linked PR is now merged. Until https://github.com/amphp/http-client-psr7/pull/10#issuecomment-2068752454 is resolved, this should also install guzzle's PSR-7.