add ChainDirectoryNamer directory namer

[guillaume-sainthillier]

This PR adds a chainable directory namer

[garak]

Please target branch 3.x for new features

[guillaume-sainthillier]

@garak done :)

[garak]

What about simple namers? Do you plan to open a PR for that too?

[guillaume-sainthillier]

@garak we can add a file namer if it make sens, IMO I don't really see the use case but why not.
For the chain directory name we can also add a way to use it by using a directory_namers config instead of my actual implementation, like this :

vich_uploader:
    # ...
    mappings:
        products:
            upload_destination: products
            directory_namers:
                # Order matter
                services: 
                    - service: vich_uploader.namer_directory_configurable
                      options:
                          directory_path: 'folder/subfolder/subsubfolder'
                    - service: Vich\UploaderBundle\Naming\PropertyDirectoryNamer
                      options:
                            property: category.slug
                    # Service ID directely
                    - 'my.directory_namer.product'
                  # Options for the ChainDirectoryNamer
                  options:  
                      separator: '/'  # optional, defaults to '/'

[guillaume-sainthillier]

@garak maybe the introduction of a new directory_namers would be cleaner than my actual implementation, what do you think?