Add implementation of OpenApiServerMockerInterface

[ybelenko]

I'm creating first implementation of OpenAPIServer\Mock\OpenApiServerMockerInterface

This class is crucial for server unit tests.
For example, when I need to test createUser API operation I need to mock valid Psr\Http\Message\ServerRequestInterface instance based on OAS3.0 Path Item Object and pass it to server handler. That's what OpenAPIServer\Mock\OpenApiServerMockerInterface::mockRequest method has been created for:

    public function mockRequest(
        string $path,
        string $method,
        ?array $parameters = null,
        ?array $requestBody = null,
        ?array $security = null,
        ?array $callbacks = null
    ): ServerRequestInterface;

I submit this issue just to show what I'm working on right now!