Searching by related entities fields
Searching by related entities fields

While we can make add columns of an related entity in the "list" actions, there is no way to search by them.

```yml
easy_admin:
    entities:
        Campaign:
            list:
                fields:
                    - property: name
                    - property: domain.name // <---
```


```yml
easy_admin:
    entities:
        Campaign:
            search:
                fields:
                    - domain.name  // <--- throwing error
```

These changes are making that it will now be possible.
What do you think about it?
yceruto/EasyAdminBundle,javiereguiluz/EasyAdminBundle,OskarStark/EasyAdminBundle,yceruto/EasyAdminBundle,OskarStark/EasyAdminBundle,yceruto/EasyAdminBundle,OskarStark/EasyAdminBundle,javiereguiluz/EasyAdminBundle