Do base64 encode for a bytes slice in gRPC calls

[andrey-gadyukov]

So, basically instead of

{
  "type_url": "type.googleapis.com/google.protobuf.BytesValue",
  "value": {
    "0": 10,
    "1": 15,
    "2": 123,
    "3": 34,
    "4": 112,
    "5": 101,
    "6": 114,
    "7": 99,
    "8": 101,
    "9": 110,
    "10": 116,
    "11": 34,
    "12": 58,
    "13": 34,
    "14": 54,
    "15": 34,
    "16": 125
  }
}

I'd like to get:

{
  "type_url": "type.googleapis.com/google.protobuf.BytesValue",
  "value": "Cg97InBlcmNlbnQiOiI2In0="
}

The Postman for instance shows the second variant.