Move export processing to a configurable plugin

Problem/Motivation

For use cases including producing Drupal distributions, certain processing is needed to change configuration on export.

Currently we make two such changes in FeaturesManager::addPackageFiles():

        // The UUID is site-specfic, so don't export it.
        if ($entity_type_id = $this->configManager->getEntityTypeIdByName($name)) {
          unset($config['data']['uuid']);
        }
        // User roles include all permissions currently assigned to them. To
        // avoid extraneous additions, reset permissions.
        if ($config['type'] == 'user_role') {
          $config['data']['permissions'] = [];
        }

An additional case is #2599268: Enable filtering of recipients on export of contact forms.

However:

• The current approach is not extensible; changes require new hard-coding.
• The current approach isn't configurable; all processing is always applied.

Processing could be implemented through new configurable assignment plugins, since assignment plugins can access and alter the full configuration array, though such a plugin wouldn't actually do any assigning of configuration to packages, so is stretching the use case of assignment plugins. Also, a processor plugin doesn't need to be weighted in relation to assigners, since the processing won't affect assignments.

We already have one assignment plugin that fits these criteria for a processor plugin: 'optional'. It doesn't create any packages or assign any configuration; it just processes the configuration collection.

So we could add a new plugin type, 'processor', and convert the 'optional' assignment plugin into a processor plugin, but this would add considerable complexity and create two different plugin types that are structurally identical. Therefore, probably best to keep this as a new assignment plugin.

Proposed resolution

Convert the current hard-coded processing in FeaturesManager::addPackageFiles() into a new configurable assignment plugin.

Remaining tasks

User interface changes

API changes

Data model changes