[5.8] Create getter for the http route middlewares

[roberto-aguilar]

This getter allows to create tests for the route middlewares, which is currently not possible because the property is protected.

For example, if you want to ensure that a route middleware has been registered, with this getter you can write:

/** @test */
public function it_registers_a_custom_route_middleware()
{
    $middlewares = resolve(\App\Http\Kernel::class)->getRouteMiddleware();

    $this->assertArrayHasKey('custom', $middlewares);
    $this->assertEquals(\App\Http\Middleware\Custom::class, $middlewares['custom']);
}

This is similar to #26268 but for the $routeMiddleware property.