Allow SchemaExtension plugins to be prioritized.

Closes #3492221

src/Annotation/SchemaExtension.php

@@ -41,6 +41,15 @@ class SchemaExtension extends Plugin {
    */
   public $schema;
 
+  /**
+   * The priority of the extension.
+   *
+   * Plugins with higher priority will be executed first.
+   *
+   * @var int
+   */
+  public $priority = 0;
+
   /**
    * SchemaExtension constructor.
    *