title: easyadmin bundle sort custom generated columns on sortable field
body: <p>Have EasyAdmin bundle config like this, and added list fields, and some of them added marked as sortable, some not. And cant find nowhere how to make sortable field:
  1) which is relation field from another entity. I.e. <code>OtherEntityName.columnName</code>;
  2) which is custom generated field from i.e. 2 fields (here is <code>fullname</code> which is made from <code>firstname</code> and <code>lastname</code> fields from the <code>EntityName</code> entity </p>
  
  <pre><code>easy_admin:
      entities:
          EntityName:
              class: App\Entity\EntityName
              label: EntityName
              list:
                  fields:
                      - {property: id, sortable: true}
                      - {property: createdAt, sortable: true}
                      - {property: fullName, sortable: true}
                      - {property: OtherEntityName.columnName, sortable: true}
  </code></pre>
  
  <p>cant find how to make it sortable, but not sorted by default. Need to make it admin action to sort, not by default like in this example: </p>
  
  <pre><code>easy_admin:
      entities:
          EntityName:
              class: App\Entity\EntityName
              label: EntityName
              list:
                  fields:
                      - {property: id, sortable: true}
                      - {property: createdAt, sortable: true}
                      - {property: fullName, sortable: true}
                      - {property: OtherEntityName.columnName, sortable: true}
                  sort: ['createdAt', 'ASC']
  </code></pre>