Use PSR-18 (and PSR-17) interfaces instead of PHP-HTTP

[JaZo]

Description

I updated the client so it requires a PSR-18 HTTP client instead of a PHP-HTTP client. As this is a breaking change, I took the liberty to also change the way our client is bound in the service provider. The client now discovers HTTP clients itself. A custom HTTP client must now be registered by customizing the container binding.

Motivation and context

PSR-18 has been released for a while and most PHP-HTTP implementations now also implement PSR-18. Both changes should also make the usage without Laravel easier.

I also implemented PSR-17 HTTP factories as the currently used factory in combination with the discovery is deprecated. Currently just a few libraries implement these interfaces, so I required nyholm/psr7 for now. Once most HTTP clients implement these interfaces (Guzzle for example is working on it), we can drop it as a dependency and leave it to the consumer to choose an implementation.

How has this been tested?

Tested with existing/updated unit tests.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

[bbrala]

Please double check the dependency, seems good otherwise.