New endpoint ListBucketObjects

Depends #1051 (closed)

Once we have basic support for buckets listing, we need a new endpoint to list the content of a bucket (named ListBucket).

It should take two parameters:

• bucket_name as path parameter
• optionally path as query parameter to specify a prefix

When path is not specified, the root folders/files are listed.

The serializer should expose basic informations:

• name is the name of the file/folder
• type is either file|folder

The user requires a new scope CephImport to use that endpoint: If the scope is missing, the endpoint simply returns a 403