Implement notification of end_of_speech in flite_port (speech_synth)

[MayamaTakeshi]

We should adapt code from wav_player. The wav_player changed to use pjmedia_event system to notify eof callback: pjproject/pjmedia/src/pjmedia/wav_player.c. So we should do something similar. (obs: this is not important for sip-lab as we only destroy the port after the AudioEndpoint is closed but we might reuse the flite_port in other projects)