Add middleware feature (extensions)

[Toilal]

You can register middlewares to customize how automapper works internally and define
global behaviors.

The following example will set 42 to any id property that would have been null.

<?php

class AnwserToUniverseMiddleware implements PropertyMiddleware
{
    public function supportsMapProperty($propertyName,
                                        $source,
                                        $destination,
                                        MappingInterface $mapping,
                                        MappingOperationInterface $operation,
                                        array $context = [])
    {
        return $propertyName == 'id';
    }

    public function mapProperty($propertyName,
                                $source,
                                $destination,
                                MappingInterface $mapping,
                                MappingOperationInterface $operation,
                                array $context = [])
    {
        $defaultValue = $mapping->getOptions()->getPropertyReader()->getProperty($destination, $propertyName);
        if ($defaultValue === NULL) {
            $mapping->getOptions()->getPropertyWriter()->setProperty($destination, $propertyName, 42);
        }
    }
}

$config->registerMiddlewares(new AnswerToLifeMiddleware());
$config->registerMapping(Employee::class, EmployeeDto::class);
$mapper = new AutoMapper($config);

// The AutoMapper can now be used as usual, but your custom mapper class will be
// called to do the actual mapping.
$employee = new Employee(NULL, 'John', 'Doe', 1980);
$result = $mapper->map($employee, EmployeeDto::class);
//$result->id = 42;

Middleware feature open up many extension capabilities, feel free to check PHPDocs
from PropertyMiddleware and
MapperMiddleware interfaces for details.

[Toilal]

I'm sorry for the big pull request, but it was in the TODO list and I really need this feature :).

Hope the implementation is what you are expecting for a middleware feature.

[BoShurik]

It does not look like a middleware for me. I think Extension name is more suitable here

[mark-gerarts]

Hi @Toilal, this is a big PR indeed! So as a head's up: it will take a while before I find the time to review this.

At a first glance I'll have to agree with @BoShurik. The usage of the before/after/skip/override constants seem like something that won't be needed in a middleware stack based on callables.

Still, lots of work went into this, and with a few changes I'll definitely merge this in master. I'll focus on the other PR's first though, before I start on this one.

[Toilal]

I see what you mean about middleware. I could refactor to provide to the middleware function a callable parameter that would perform the default mapping operation when called, so that middleware implementation can invoke it at the start of the method, at the end of the method, or not at all.

This would then make the "before/after/override/skip" thing useless.

[Toilal]

I'm closing this pull request in favor or #47