Create read model API

I propose the bundle will use `ReadModelService` as such:

```php
// IdentityRepository
function getIdentity($identityId, $instituteId)
{
  $identityData = $this->readModelService->read(
    '/institute/%s/identity/%s',
    [$instituteId, $identityId]
  );

  // beberlei/assert
  \Assert\Assertion::keyExists($identityData, 'id');
  \Assert\Assertion::string($identityData['id']);
  \Assert\Assertion::keyExists($identityData, 'name_id');
  \Assert\Assertion::string($identityData['name_id']);

  $identity = new \Surfnet\StepupMiddlewareClientBundle\ReadModel\Identity;
  $identity->id = $identityData['id'];
  $identity->nameId = $identityData['id'];
```