Implement support for loop in start_speech_synth

[MayamaTakeshi]

We should make it similar to pjmedia_wav_player_port. So actually, we will implement loop support and then allow it to be disabled with

no_loop: true

So we should allow options (flags in the documentation) to be set to PJMEDIA_SPEECH_NO_LOOP (equivalent of PJMEDIA_FILE_NO_LOOP). See: https://docs.pjsip.org/en/latest/api/generated/pjmedia/group/group__PJMEDIA__FILE__PLAY.html