log_streaming: make base64 format clearer

This way it's harder to miss this.

protos/log_streaming/log_streaming.proto

@@ -34,7 +34,7 @@ message LogStreamingRawResponse {
 
 // Raw logging data type
 message LogStreamingRaw {
-    string data = 1; // Ulog file stream data encoded as base64
+    string data_base64 = 1; // Ulog file stream data encoded as base64
 }
 
 // Result type.