Lorawan-stack: Inconsistent enum rendering in JSON responses

Summary

References: https://github.com/TheThingsNetwork/lorawan-stack/issues/2047

Values returned by NS for enums within the mac_settings field are inconsistent. Some for some NS returns raw values and for others a string representation is returned.

Steps to Reproduce

1. Set mac_settings.rx1_delay.value

// request
{
   "end_device":{
      "mac_settings":{
         "rx1_delay":{
            "value":5 // or 'RX_DELAY_5'
         }
      }
   },
   "field_mask":{
      "paths":[
         "mac_settings.rx1_delay.value",
      ]
   }
}

// response
{
   ...
   "mac_settings":{
      "rx1_delay":{
         "value":5
      },
   }
}

// Note, regardless of what you send as the value for mac_settings.rx1_delay.value you always get the raw enum value.

1. Set mac_settings.ping_slot_periodicity.value

// request

{
   "end_device":{
      "mac_settings":{
         "ping_slot_periodicity":{
            "value": 1 // or 'PING_EVERY_2S'
         }
      }
   },
   "field_mask":{
      "paths":[
         "mac_settings.ping_slot_periodicity.value"
      ]
   }
}

// response

{
   "mac_settings":{
      "ping_slot_periodicity":{
         "value":"PING_EVERY_2S"
      }
   }
}

// Again, regardless of what you send you always get the string representation of the value.

What do you see now?

Inconsistent values for mac_settings enum fields

What do you want to see instead?

All values should be returned in the same format (raw/string)

Environment

v3.7.0-rc2

How do you propose to implement this?

I guess first we have to agree on the format. Then it is the matter of adjusting https://github.com/TheThingsNetwork/lorawan-stack/blob/master/api/lorawan.proto and https://github.com/TheThingsNetwork/lorawan-stack/blob/master/pkg/ttnpb/lorawan.go

Can you do this yourself and submit a Pull Request?

Yes, but need input from @rvolosatovs @johanstokking @htdvisser