add simple configuration for bind address of outgoing sockets

[mzhou]

when specified like --outgoing-addr=1.2.3.4:0, outbound sockets will be bound to 1.2.3.4 before connecting. there are cases where a specific port may be desired instead of 0 for auto-select, but most uses cases call for 0, so i can add a fallback to parse the argument as an IpAddr (and constructing a SocketAddr with port 0) if parsing as SocketAddr fails. just wanted to see what you think of the general concept first though

[dizda]

Hey there,

I'm not sure to understand the purpose of this, could you elaborate or give me an example?

Much appreciated.
Thank you.

[Shados]

@dizda I'm not the PR author, but at a guess the intent is to let you select which IP is used as the source IP, when run on a machine with multiple IP addresses.

[dizda]

Sorry about the delay.
This is a legitimate feature, thanks for adding this.
There's some conflicts to fix before I can merge this, could you have a look at it please @mzhou ?