Introduce "user type" level in reate limits configuration

[mcucchi9]

This PR introduce a "user type" top-level in rate limits configuration to allow for the specifications of rate limits for authenticated users (key: auth) and non-authenticated users (key: anon).
Both should be specified in order to enforce rate limits for the two classes of users.
With this feature implemented, the configuration file should like the following:

auth:
  default:
    post:
      api: <limits>
      ui: <limits>
    get:
      api: <limits>
      ui: <limits> 
    ...
  /processes/{process_id}/execution:
    default:
      post:
        api: <limits>
        ui: <limits>
    <process_id_1> (e.g. "reanalysis-era5-single-levels"):
      post:
        api: <limits>
        ui: <limits> 
     ...    
anon:
  default:
    post:
      api: <limits>
      ui: <limits>
    get:
      api: <limits>
      ui: <limits> 
    ...
  /processes/{process_id}/execution:
    default:
      post:
        api: <limits>
        ui: <limits>
    <process_id_1> (e.g. "reanalysis-era5-single-levels"):
      post:
        api: <limits>
        ui: <limits> 
     ... 

[mcucchi9]

CLOSED -> moved to #267