Add SIP user-agent library for VoIP (AUD-607)

[renne]

The ESP32 is a great SoC for wireless VoIP phones. Please add a IETF RFC 3261-compliant SIP user-agent library for audio handling.

[jason-mao]

@renne Cool. We are already have a SIP demo for internal test.
We plan to integrate SIP to ADF after next release(next release will be on next month).

[jason-mao]

#57

[doanerock]

Will there be ZRTP and SRTP or is it too soon to ask?

[bwannduke]

Is the SIP demo still close to release?

[tommichaud]

Jason - Any update on the SIP supported release?

[ahhfzhang]

SIP user-agent library and VoIP demo has been released.

[Niek]

Reference: https://github.com/espressif/esp-adf/tree/master/examples/advanced_examples/voip