Handles google.protobuf.Value fields by delegating to a StringHandler

[rajuGT]

• There are two concerns with google.protobuf.Value type
  1. It has cyclic reference - https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/struct.proto#L94
  2. oneof usage; i.e. It has no specific type information, every message can be of different type - https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/struct.proto#L64

This handler treats Value as a plain string:

• Converts between String and Value
• Ignores nested structures (STRUCT_VALUE, LIST_VALUE)
• Useful as a lightweight workaround to recursion in google.protobuf.Value

[rajuGT]

#67