Use IETF application error codes for STOP_SENDING frames. Add ietf_error_code member to QuicStopSendingFrame, and use IetfResetStreamErrorCodeToRstStreamErrorCode() and RstStreamErrorCodeToIetfResetStreamErrorCode() to convert to and from QuicRstStreamErrorCode. This closely mirrors behavior of QuicRstStreamFrame. Protected by FLAGS_quic_reloadable_flag_quic_stop_sending_uses_application_error_code. PiperOrigin-RevId: 330521460 Change-Id: Ifc69cca79e47058bc6c1eac91661380eab4b8e02