SessionCacheContext class should implement CacheContextInterface

Found this following 📌 Enable service autoconfiguration for core event subscribers Fixed

If we enable autoconfiguration, that means we can automatically add the cache.context tag to services that implement a specific interface. But as I found over there, not all cache contexts actually implement CacheContextInterface.

I think we should start throw deprecation if cache contexts does not implement context interface

+++ b/core/tests/Drupal/KernelTests/Core/Cache/CacheCollectorTest.php
@@ -60,4 +62,15 @@ public function providerTestInvalidCharacters() {
+   * Tests that cache contexts implement the right interfaces.
...
+  public function testContextImplementsInterface() {
+    $cache_contexts = \Drupal::service('service_container')->findTaggedServiceIds('cache.context');
+    foreach (array_keys($cache_contexts) as $context) {
+      $service = \Drupal::service($context);
+      $this->assertTrue($service instanceof CalculatedCacheContextInterface || $service instanceof CacheContextInterface);

This test still makes sense but needs to throw deprecation if collector discovers cache context without interface

+++ b/core/lib/Drupal/Core/Cache/Context/RequestStackCacheContextBase.php
@@ -11,7 +12,7 @@
-abstract class RequestStackCacheContextBase {
+abstract class RequestStackCacheContextBase implements CacheContextInterface {

The change in #18 is BC break so collector should care about it

Looking at usage of CalculatedCacheContextInterface there's only one check for interface in \Drupal\Core\Cache\Context\CacheContextsManager::getLabels()