[go-to-protobuf] Quote tag cast{key,value,type}

When using a `cast{key,value,type}` that was injected via struct tag, we
need to make sure to quote the value when transfering it over to proto
tags.  Otherwise, it'll come through as unquoted, resulting in invalid
proto.

This was previously not a problem, since all values of `castkey` and
`casttype` were actually coming from the auto-injecting code which deals
with maps and aliases, which does correctly quote values.

cmd/libs/go2idl/go-to-protobuf/protobuf/generator.go

@@ -598,7 +598,7 @@ func protobufTagToField(tag string, field *protoField, m types.Member, t *types.
 			protoExtra[parts[0]] = parts[1]
 		case "casttype", "castkey", "castvalue":
 			parts[0] = fmt.Sprintf("(gogoproto.%s)", parts[0])
-			protoExtra[parts[0]] = parts[1]
+			protoExtra[parts[0]] = strconv.Quote(parts[1])
 		}
 	}