FEATURE: Fully support PSR-18

[mhsdesign]

i just noticed that our \Neos\Flow\Http\Client\RequestEngineInterface matches exactly the \Psr\Http\Client\ClientInterface so if we deprecate ours and make it extend the psr standard our browser would be psr compatible and one could use the GuzzleHttp client ^^

@mficzel added:

• The \Neos\Flow\Http\Client\Browser already implements \Psr\Http\Client\ClientInterface

We somehow missed to add

"provide": {
        "psr/http-client-implementation": "1.0"
    },

to the composer json of flow

Also the Flow.httpfactories package should probably also provide:

"provide": {
        "psr/http-factory-implementation": "1.0",
        "psr/http-message-implementation": "1.0"
    },