Rate limits configuration for unauthenticated users

[mcucchi9]

This PR introduce an "unauthenticated" top-level key in rate limits configuration to allow for the specifications of overall rate limits for unauthenticated users.
With this feature implemented, the configuration file should like the following:

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> 
   ...    
unauthenticated:
  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> 
     ... 

[keul]

Format LGTM to me