ISSUE-28: Lazily build the handler

[doppiogancio]

    public function testLazyBuiltHandler(): void
    {
        $this->handlerBuilder->addRoute(
            $this->routeBuilder->new()
                ->withMethod('PATCH')
                ->withPath('/lazy/builder')
                ->withResponse(new Response(123))
                ->build(),
        );

        $response = $this->getMockedClient()->request('PATCH', '/lazy/builder');
        $this->assertEquals(123, $response->getStatusCode());
    }

[guhemama]

Looks great!

[doppiogancio]

Only after doing some tests, I recognized that no change is needed in order to have the "lazy builder", it was already like this. I will keep the tests, so that can be used as examples.