New migrate field discovery service for fields and field instances

Summary
A new service has been added ('migrate_drupal.field_discovery') to simplify the process of detecting and adding appropriate field process arrays for content migrations of Drupal 6 nodes and Drupal 7 fieldable entities. This service contains the method addFields(MigrationInterface $migration, $entity_type, $bundle = '') and detects and adds the appropriate field process arrays to the passed in migration.

Before

     // Read all field instance definitions in the source database.
    $fields = [];
    try {
      $source_plugin = static::getSourcePlugin('d6_field_instance');
      $source_plugin->checkRequirements();

      foreach ($source_plugin as $row) {
        $fields[$row->getSourceProperty('type_name')][$row->getSourceProperty('field_name')] = $row->getSource();
      }
    }
    catch (RequirementsException $e) {
      // If checkRequirements() failed then the content module did not exist and
      // we do not have any fields. Therefore, $fields will be empty and
      // below we'll create a migration just for the node properties.
    }

    try {
        /** @var \Drupal\migrate\Plugin\Migration $migration */
        $migration = \Drupal::service('plugin.manager.migration')->createStubMigration($values);
        if (isset($fields[$node_type])) {
          foreach ($fields[$node_type] as $field_name => $info) {
            $field_type = $info['type'];
            try {
              $plugin_id = $this->fieldPluginManager->getPluginIdFromFieldType($field_type, ['core' => 6], $migration);
              if (!isset($this->fieldPluginCache[$field_type])) {
                $this->fieldPluginCache[$field_type] = $this->fieldPluginManager->createInstance($plugin_id, ['core' => 6], $migration);
              }
              $this->fieldPluginCache[$field_type]
                ->processFieldValues($migration, $field_name, $info);
            }
            catch (PluginNotFoundException $ex) {
              try {
                $plugin_id = $this->cckPluginManager->getPluginIdFromFieldType($field_type, ['core' => 6], $migration);
                if (!isset($this->cckPluginCache[$field_type])) {
                  $this->cckPluginCache[$field_type] = $this->cckPluginManager->createInstance($plugin_id, ['core' => 6], $migration);
                }
                $this->cckPluginCache[$field_type]
                  ->processCckFieldValues($migration, $field_name, $info);
              }
              catch (PluginNotFoundException $ex) {
                $migration->setProcessOfProperty($field_name, $field_name);
              }
            }
          }
        }
        $this->derivatives[$node_type] = $migration->getPluginDefinition();
      }
    }

After:

    try {
      foreach ($node_types as $row) {
        /** @var \Drupal\migrate\Plugin\Migration $migration */
        $migration = \Drupal::service('plugin.manager.migration')->createStubMigration($values);
        \Drupal::service('migrate_drupal.field_discovery')->addFields($migration, 'node', $node_type);
        $this->derivatives[$node_type] = $migration->getPluginDefinition();
      }
    }