Added Query Hints support to Entity form type that use query builder

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [none]
| License       | MIT
| Doc PR        | [will add later on]

Adds an optional "query_hints" array to be passed to "entity" form types, to be used when generating the Query object to retrieve objects from storage.

I've added a basic test, that just validates the scenario where the input type for the "query_hints" is incorrect. I wanted to add a test to validate the working scenario, but my phpunit/doctrine knowledge only goes so far, so if anyone could give me a help with it, I'd really appreciate.