Declare most classes as final

Most of our classes are not meant to be extended, and to prevent possible
future breaking change we prefer to prevent extending from them and instead
promote the discussion about use cases and potential extension points.

Author: PowerKiKi

README.md

@@ -414,7 +414,7 @@ use GraphQL\Doctrine\Annotation as API;
  *     @API\Filter(field="custom", operator="GraphQLTests\Doctrine\Blog\Filtering\Search", type="string")
  * })
  */
-class Post extends AbstractModel
+final class Post extends AbstractModel
 ```
 
 #### Custom sorting
@@ -435,7 +435,7 @@ use GraphQL\Doctrine\Annotation as API;
  * @ORM\Entity
  * @API\Sorting({"GraphQLTests\Doctrine\Blog\Sorting\UserName"})
  */
-class Post extends AbstractModel
+final class Post extends AbstractModel
 ```
 ## Limitations
 

src/Annotation/Argument.php

@@ -21,6 +21,6 @@
  *     @Attribute("defaultValue", required=false, type="mixed"),
  * })
  */
-class Argument extends AbstractAnnotation
+final class Argument extends AbstractAnnotation
 {
 }

src/Annotation/Exclude.php

@@ -12,6 +12,6 @@
  * @Annotation
  * @Target({"METHOD"})
  */
-class Exclude
+final class Exclude
 {
 }

src/Annotation/Field.php

@@ -13,7 +13,7 @@
  * @Annotation
  * @Target({"METHOD"})
  */
-class Field
+final class Field
 {
     /**
      * @var string

src/Annotation/Filter.php

@@ -15,7 +15,7 @@
  * @Annotation
  * @Target({"ANNOTATION"})
  */
-class Filter
+final class Filter
 {
     /**
      * Name of the field on which to apply the operator

src/Annotation/Filters.php

@@ -10,7 +10,7 @@
  * @Annotation
  * @Target({"CLASS"})
  */
-class Filters
+final class Filters
 {
     /**
      * List of all custom filters

src/Annotation/Input.php

@@ -22,6 +22,6 @@
  *     @Attribute("defaultValue", required=false, type="mixed"),
  * })
  */
-class Input extends AbstractAnnotation
+final class Input extends AbstractAnnotation
 {
 }

src/Annotation/Sorting.php

@@ -10,7 +10,7 @@
  * @Annotation
  * @Target({"CLASS"})
  */
-class Sorting
+final class Sorting
 {
     /**
      * FQCN of PHP class implementing `SortingInterface`

src/DefaultFieldResolver.php

@@ -15,7 +15,7 @@
  * A field resolver that will allow access to public properties and getter.
  * Arguments, if any, will be forwarded as is to the method.
  */
-class DefaultFieldResolver
+final class DefaultFieldResolver
 {
     public function __invoke($source, $args, $context, ResolveInfo $info)
     {

src/Definition/EntityIDType.php

@@ -11,7 +11,7 @@
 /**
  * A specialized ID type that allows to fetch entity from DB
  */
-class EntityIDType extends IDType
+final class EntityIDType extends IDType
 {
     /**
      * @var EntityManager