Implement Transaction/operation builder functional layer covering another complex operation building scenarios

• support for operation content (i.e. memo/post content) encryption/decryption
• witness_set_operation uses custom serialization of witness properties. This makes tedious usage of this operation. Best if we could encapsulate common C++ code responsible for it inside Wax C++ layer and expose it to TS/Python
• simplify account JSON metadata update (now pure operation must be built). Maybe dedicated support for predefined properties should be implemented (similary to comment json metadata)
• Add dedicated support for account authority management