Investigate removal of base64 encoding in `Handle...Message` functions

[mostafa]

Currently all the JSON-marshaled messages are encoded to base64 and then these messages are decoded from base64 and marshalled back to JSON. This takes time and has performance overhead. On the other hand, different message types might have different fields that might be bytes (binary value). This should be investigated to see if we can remove this encoding/decoding altogether and pass the data as v1.Struct (as a JSON object) without all the hassle.