Add Send short data API

A new feature of LINE Planet, Send short data, has been released.

Send short data API allows the application to send data to the other party after the call is connected. Currently, only 1:1 calls are supported. When group calls are supported in the future, you can send data to people who have joined the room.

Because it is a transaction-based transmission method, it can transmit data up to 700 bytes. During a transaction, retransmissions will be performed several times if the transmission fails. However, if all retransmissions fail due to network conditions (if no response is received from the other party), the failure result is notified to the application, and the application must take an appropriate action.