Allow creating directory subfolders by giving the path in the config

[syffer]

---

name: ⚙ Improvement
about: Allowing to set the image subfolders directly from the yaml configuration (without having to rely on property accessor)

Improvement

Q	A
New Feature	yes
RFC	no
BC Break	no

Summary

This pr is a proposal to add a new directory namer, which allow to configure the image subfolders directly from the yaml configuration.

This would allow to use multiple vich mapping on the same storage, without

• having to duplicate the storage configuration for each mapping
• be forced to use the PropertyDirectoryNamer, which add a method solely for this purpose

For example, it would allow to do the following:

vich_uploader:
    # ...

    storage: flysystem

    mappings:
        logo:
            uri_prefix: '%app.uploads.uri_prefix%'
            upload_destination: storage.uploads.cdn
            directory_namer:
                service: vich.namer_directory_configurable
                options:
                    directory_path: custom/logos
            namer:
                service: Vich\UploaderBundle\Naming\SmartUniqueNamer

        map:
            uri_prefix: '%app.uploads.uri_prefix%'
            upload_destination: storage.uploads.cdn
            directory_namer:
                service: vich.namer_directory_configurable
                options:
                    directory_path: custom/maps
            namer:
                service: Vich\UploaderBundle\Naming\SmartUniqueNamer

flysystem:
    storages:
        storage.uploads.cdn:
            adapter: 'gcloud'
            visibility: !php/const League\Flysystem\GoogleCloudStorage\PortableVisibilityHandler::NO_PREDEFINED_VISIBILITY
            options:
                client: 'app.uploads_cdn.gcloud_client_service'
                bucket: '%uploads.cdn.gcp.storage.bucket_name%'
                prefix: '%uploads.cdn.gcp.storage.path_prefix%'

This would also take care of #1377