rtpproxy: add rtpp_bind_local support for the offer/answer/engage.

[sobomax]

Summary

This PR adds more flexible control over socket that RTPProxy picks for a new or existing media session. The existing mechanism with I/E designators is a bit crude and limiting.

Details

When rtpp_bind_local AVP is set, its value would be provided to the rtpproxy with "l" modifier, allowing proper address to be selected for the session.

Solution

This can be used either globally by setting the rtpproxy module parameter:

modparam("rtpproxy", "rtpp_bind_local_avp", "$socket_out(ip)")

Or on logic level by setting up the AVP:

route[] {
...
   $avp(rtpp_bind_local) = '1.2.3.4';
   rpproxy_engage();
...

Compatibility

Should be none, it's a new API.

Closing issues

None