Provide descriptions and types for the RBD configuration options
Currently, the dashboard needs to add additional information (a display name, description and type) to the RBD configuration options to be able to present those options in a user friendly way. Ideally, this information should be returned by Ceph (librados) alongside the current data (name (key), source and value).
It seems that the type for those options is already returned for global configuration options, but isn't passed through the `pool_metadata_get` and `image.metadata_get` methods offered by librados. The types added in the frontend differ from those types available for global configuration in that they describe the units for those options instead of the basic data types. This enables the frontend to dynamically create the input fields and validate them appropriately.
By adding more of this information, either in the dashboard itself or returned by librados, the dashboard could dynamically create those options, validate them accordingly and support the user with the correct units for those options automatically.
The benefit of having those information returned by librados is that there wouldn't be a need to update, create or maintain the options in the frontend anymore, if an option was added, removed or changed, which would results in less to none maintenance work for the RBD configuration options implementation in the dashboard. Other mgr modules or consumers of librados would also benefit from having this information available.