Added sonata_type_model_autocomplete form type

Related to sonata-project/SonataAdminBundle#1764

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -
What is ``to_string_callback`` is provided by the Admin instance