Add PSR-7 support

[Art4]

This PR will add PSR-7 HTTP message support to Requests. This means that Requests will be able to process any PSR-7 Psr\Http\Message\RequestInterface implementation and response with a PSR-7 Psr\Http\Message\ResponseInterface implementation.

Pull Request Type

• [x] I have checked there is no other PR open for the same change.

This is a:

• [x] New feature

Context

@jrfnl stated in https://github.com/WordPress/Requests/issues/320#issuecomment-1236301755 that PSR-7 support would be a useful addition. Therefore I started with the implementation as a draft. Since I don't have any experience in contributing to Requests yet (I've read the .github/CONTRIBUTING.md), I appreciate early feedback to avoid gross errors early on.

Detailed Description

The plan is that the PSR-7 implementation will be built as a wrapper around the existing classes in the WpOrg\Requests\Psr namespace. This way there should be no breaking changes and the usage is optional. This is also important because PSR-7 has some limitations, such as missing multible requests.

Long term goal is as I mentioned here a PSR-18 implementation to be able to use Requests as a PSR-18 HTTP client. However, this will require PHP 7.0+ and is therefore not part of this PR. Nevertheless, this PR already includes a class that is compatible with PSR-18 and PSR-17 RequestFactory, but does not yet rely on the interfaces. This must be implemented in a future PR after bumping the PHP version to 7.0+.

Work progress

• [x] Implement request factory compatible to Psr\Http\Message\RequestFactoryInterface
• [x] Implement Psr\Http\Message\UriInterface
• [x] Implement Psr\Http\Message\RequestInterface
• [x] Implement Psr\Http\Message\StreamInterface
• [x] Implement stream factory compatible to Psr\Http\Message\StreamFactoryInterface
• [ ] Implement Psr\Http\Message\ResponseInterface
• [ ] Implement HTTP client compatible to Psr\Http\Client\ClientInterface

Quality assurance

• [x] This change does NOT contain a breaking change (fix or feature that would cause existing functionality to change).
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have added unit tests to accompany this PR.
• [x] The (new/existing) tests cover this PR 100%.
• [x] I have (manually) tested this code to the best of my abilities.
• [ ] My code follows the style guidelines of this project.

Documentation

For new features:

• [ ] I have added a code example showing how to use this feature to the examples directory.
• [ ] I have added documentation about this feature to the docs directory. If the documentation is in a new markdown file, I have added a link to this new file to the Docs folder README.md file.