Move SRTP setting in PJSUA and PJSUA2 to account setting

[nanang]

Currently SRTP crypto and keying method is configurable via callback, i.e: on_create_media_transport_srtp() in PJSUA or onCreateMediaTransportSrtp() in PJSUA2 (only crypto). And after some review and internal discussions, we decided to deprecate the callback and move the settings to account config, here are some reasons:

• SRTP setting type used in callback param is pjmedia_srtp_setting, which has some fields that are PJMEDIA specifics, e.g: close_member_tp, cb, user_data. So this ticket will also create a new SRTP settings specifically for PJSUA and PJSUA2.
• other media transports are configurable via account setting (instead of PJSUA/PJSUA2 callback).