Simplify signature of the low-level API

[Art4]

At the moment we have these 7 methods to interact with the low-level API:

    /**
     * Create and send a GET request.
     */
    public function requestGet(string $path): bool;

    /**
     * Create and send a POST request.
     */
    public function requestPost(string $path, string $body): bool;

    /**
     * Create and send a PUT request.
     */
    public function requestPut(string $path, string $body): bool;

    /**
     * Create and send a DELETE request.
     */
    public function requestDelete(string $path): bool;

    /**
     * Returns status code of the last response.
     */
    public function getLastResponseStatusCode(): int;

    /**
     * Returns content type of the last response.
     */
    public function getLastResponseContentType(): string;

    /**
     * Returns the body of the last response.
     */
    public function getLastResponseBody(): string;

The first 4 methods will send a request to the server and save the response in the Client. The last 3 methods can now be used to get the response details. This has some downsides:

1. If we send multiple requests (like we do in the mid-level API AbstractApi::retrieveData()), only the latest response will be available in the client. This leads to possible race conditions: There is no way to guarantee, that the last response in Client really belongs to our request.
2. We have to call the client 3 times to retrieve all response data. This is a code smell and should be replaced with a small object.

Proposal

I propose to introduce a new Request containing the method, path, content type and body and a new Response interface, containing the status code, content type and body.

interface Request
{
    /**
     * Returns the http method.
     */
    public function getMethod(): string;

    /**
     * Returns the path with optional attached query string.
     */
    public function getPath(): string;

    /**
     * Returns content type.
     */
    public function getContentType(): string;

    /**
     * Returns the body content.
     */
    public function getContent(): string;
}

interface Response
{
    /**
     * Returns status code.
     */
    public function getStatusCode(): int;

    /**
     * Returns content type.
     */
    public function getContentType(): string;

    /**
     * Returns the body content.
     */
    public function getContent(): string;
}

The client methods could then simplified into one method.

public function request(Request $request): Response;

[Art4]

Another point that I would like to improve is the "content type guessing" based on the path in the clients. In NativeCurlClient and Psr18Client we have the following code to determine the content type:

        if ($this->isUploadCall($path)) { // (false !== strpos($path, '/uploads.json')) || (false !== strpos($path, '/uploads.xml'))
            $httpHeaders[] = 'Content-Type: application/octet-stream';
        } elseif ('json' === substr($tmp['path'], -4)) {
            $httpHeaders[] = 'Content-Type: application/json';
        } elseif ('xml' === substr($tmp['path'], -3)) {
            $httpHeaders[] = 'Content-Type: text/xml';
        }

This method is very speculative and uncertain. The client should not make any assumptions about the request.

Instead we should add a way to provide the content type via the request() method. It makes sense to bundle the 4 parameters (method, path, content-type, content) in a simplified request interface that is compatible to PSR-7. I will update have updated the proposal.

Usage of the low-level API could then work like this:

$response = $this->getHttpClient()->request(XmlRequest::put(
    '/issues/' . $id . '.xml',
    ['issue' => $params],
));

$array = XmlSerializer::createFromString($response->getContent())->toArray();