Feature/ajax choice type

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [no]
Unit test passes: [yes]
Behat scenarios passes: [yes]
Checkstyle issues: [no]*
ChangeLog updated: [no]
Documentation PR: []
Fixes the following jira:


This PR adds a new "pim_ajax_entity" type. Repositories for entities used in this type should implement the OptionRepositoryInterface interface.

For the moment, the new field type is only used in multiple and single select attributes fields in the product form, but it also could (should) be used in the form grid filters, and eventually for other selects.