Implement static caching around SchedulerManager::getPlugins()

Currently, when a provider is passed, the persistent cache is bypassed. With this change, we always use the static cache if present, then fallback to the persistent cache. The provider filtering is done after the fact. I think this makes more sense than bypassing the persistent cache entirely when a $provider is passed.

Attaching a static patch for use with composer.

I think this is an interesting fix, the getPlugins() method is called a lot and currently each call to that method will make a request to the cache backend (in my case Redis), For example, 8 cache get calls on a simple add to cart request.

I'm actually wondering if the persistent cache fully makes sense here, because doGetPlugins() calls $this->getPluginDefinitions();Which gets the definitions from the plugin manager:

$this->pluginManager->getDefinitions();

This invokes the getDefinitions() method on the DefaultPluginManager which has the following logic:

/**
   * {@inheritdoc}
   */
  public function getDefinitions() {
    $definitions = $this->getCachedDefinitions();
    if (!isset($definitions)) {
      $definitions = $this->findDefinitions();
      $this->setCachedDefinitions($definitions);
    }
    return $definitions;
  }

So the plugin definitions already have static cache + persistent cache handling. So we define our own cache on top of the core cache handling.

Our own persistent cache probably still makes sense otherwise we'll perform the following logic over and over:

    $definitions = $this->getPluginDefinitions();
    foreach ($definitions as $definition) {
      $plugin = $this->pluginManager->createInstance($definition['id']);
      $dependency = $plugin->dependency();
      // Ignore plugins if there is a dependency module and it is not enabled.
      if ($dependency && !$this->moduleHandler->moduleExists($dependency)) {
        continue;
      }
      $this->plugins[$plugin->entityType()] = $plugin;
    }