What's in this PR?

This PR adds the support for multiple form metadata loaders. The different FormMetadataLoader classes can be tagged in the associated services.xml files as sulu_admin.form_metadata_loader in order to get injected automatically in the FormMetadataProvider. The different FormMetadataLoader classes must implement the Interface FormMetadataLoaderInterface.

Furthermore this PR changes the loading and caching mechanism of form metadata. The FormXmlLoader returns now an instance of LocalizedFormMetadataCollection, which contains a FormMetadata object for each locale. Instead of the old metadata objects, the newer ones are used now. The old metadata classes are now marked as deprecated.

The XmlFormMetadataLoader implements the FormMetadataLoaderInterface and provides the functionality to load and cache the form metadata. This was done previously in the FormMetadataProvider.
The StructureFormMetadataLoader is added as well in this PR and also implements the FormMetadataLoaderInterface. The StructureLoader is responsible for caching the structure metadata.

The class FormMetadataMapper handles the conversation between old and new metadata objects and is used in the XmlFormMetadataLoader, StructureFormMetadataLoader and FormXmlLoader.

Why?

Enable to use a custom metadata loader provided from the form bundle (see PR sulu/SuluFormBundle#192) . The functionality can also be useful for other bundles that want to provide their own metadata loader.

The modification to use the new metadata classes in the FormXmlLoader is necessary to avoid the usage of the old metadata objects in new loaders.

Example Usage

Add a service that implements the FormMetadataLoaderInterface. Afterwards tag this service in the services.xml file as sulu_admin.form_metadata_loader.
Example:

 <service
            id="sulu_admin.xml_form_metadata_loader"
            class="Sulu\Bundle\AdminBundle\Metadata\FormMetadata\XmlFormMetadataLoader"
>
            <argument type="service" id="sulu_admin.form_metadata.form_xml_loader"/>
            <argument>%sulu_admin.forms.directories%</argument>
            <argument>%sulu.cache_dir%/forms</argument>
            <argument>%kernel.debug%</argument>
            <tag name="sulu_admin.form_metadata_loader"/>
            <tag name="kernel.cache_warmer" />
</service>

BC Breaks/Deprecations

Describe BC breaks/deprecations here. (remove this section if not needed)

To Do

• Create a documentation PR
• Add breaking changes to UPGRADE.md