Create read model API

Bundle will use `IdentityService` as such:

```php
// IdentityService
function get($id, $institution)
{
  $identityData = $this->clientService->get($id, $institution);
  $identity = ReadModel\Identity::fromData($identityData);

  $violations = $this->validator->validate($identity);

  if (count($violations) > 0) throw_and_log();
```
please adjust the docblock terminology as well :wink: