Resolve #2453835 "Allow non intrinsic implementation dependent"

Closes #2453835

core/lib/Drupal/Core/Cache/Context/CacheContextOptimizableInterface.php

+<?php
+
+namespace Drupal\Core\Cache\Context;
+
+/**
+ * Allows cache contexts to influence how cache contexts are optimized.
+ */
+interface CacheContextOptimizableInterface {
+
+  /**
+   * Returns whether a cache context value has variations.
+   *
+   * For example, the timezone cache context can define itself as global
+   * if a site does not have configurable timezones and the language contexts
+   * are global on a site that is not multilingual.
+   *
+   * @return bool
+   *   TRUE if the context has more than one possible variations.
+   */
+  public function hasVariations(): bool;
+
+  /**
+   * Returns parent contexts for this context.
+   *
+   * By default, parents are derived based on the cache context name, user is
+   * a parent of user.permissions. This allows e.g. the user.roles context to
+   * expand on this and indicate that user.permissions is also a valid
+   * parent/replacement.
+   *
+   * Note that if implementing this, it is necessary to also include parents of
+   * the parent contexts, e.g. user.roles must provide both user.permissions and
+   * user.
+   *
+   * @return array
+   *   A list of parents cache contexts.
+   */
+  public function getParentContexts(): array;
+
+}