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.
• media transport settings are usually configurable via account setting (instead of PJSUA/PJSUA2 callback), so it'd better to use the same pattern for SRTP.

Here are sample codes to enable DTLS-SRTP in outgoing SDP offer:

• using PJSUA:

   acc_cfg.srtp_opt.keying_count = 2;
   acc_cfg.srtp_opt.keying[0] = PJMEDIA_SRTP_KEYING_DTLS_SRTP;
   acc_cfg.srtp_opt.keying[1] = PJMEDIA_SRTP_KEYING_SDES; // optional, as a fallback for handling incoming call using SRTP with SDES
  

• using PJSUA2:

  acc_cfg.mediaConfig.srtpOpt.keyings.push_back(PJMEDIA_SRTP_KEYING_DTLS_SRTP);
  acc_cfg.mediaConfig.srtpOpt.keyings.push_back(PJMEDIA_SRTP_KEYING_SDES); // optional, as a fallback for handling incoming call using SRTP with SDES
  

[nanang]

In 5755:

Close #2100:

• Added new APIs:
  • PJMEDIA: pjmedia_srtp_enum_crypto(), pjmedia_srtp_enum_keying()
  • PJSUA: pjsua_config.srtp_opt, pjsua_acc_config.srtp_opt, pjsua_srtp_opt_default()
  • PJSUA2: AccountMediaConfig::srtpOpt, Endpoint::srtpCryptoEnum()
• Deprecated PJSUA callback on_create_media_transport_srtp() (not removed yet, just warnings).
• Slightly refactored SRTP code:
  • Fixed potential issue with on_create_media_transport_srtp(), some PJSUA internal values in pjmedia_srtp_setting may be overridden by app.
  • Fixed few issues in SRTP and keying mechanism, e.g: premature local SDP modification (it should be done after verification).
  • Potential minor backward compatibility issue: default value of pjmedia_srtp_setting.crypto_count is now zero, previously it was initialized with all crypto via pjmedia_srtp_setting_default(), actually zero and all cryptos in this setting semantically are the same.