tuic-server: add outbound config: ip_mode(ipv4/v6 preference), bind_ipv4/v6 and bind_device

[fokx]

tuic-server: allow outbound config: ip_mode(ipv4/v6 preference), bind_ipv4/v6 and bind_device

This commit should fix #26

This is a WIP. Outbound rules other than default are not used until ACL is implemented.

[fokx]

This is inspired by Hysteria's outbound settings.
https://v2.hysteria.network/docs/advanced/Full-Server-Config/#outbounds

Considering hysteria uses yaml whereas tuic uses toml, the current configuration syntax is perhaps not optimal. Feel free to modify.

[Itsusinn]

I used YAML for my several former programs. It works great whether the configuration is complex or not. TUIC is my first trying of TOML. TOML is very suitable for the original TUIC(minimalist implementation) . However it becomes worse the adding more options to configuration

[fokx]

From https://toml.io/en/ :

TOML prioritizes humans
TOML aims to be a minimal configuration file format that:
is easy to read due to obvious semantics
maps unambiguously to a hash table
is easy to parse into data structures in a wide variety of languages

For toml, Succinctness is not a priority. Luckily, the outbound configuration format is not too complex. Many fields are optional.

[outbound.prefer_v4]
type = "direct"
# Optional
ip_mode = "prefer_v4"
# Optional
bind_ipv4 = "1.2.3.4"
# Optional
bind_ipv6 = "0:0:0:0:0:ffff:0102:0304"
# Optional
bind_device = "eth1234"

[outbound.through_socks5]
# TODO: `socks5` outbound is not implemented yet
type = "socks5"
# SOCKS5 proxy addr
addr = "127.0.0.1:1080"
# Optional
username = "optional"
# Optional
password = "optional" 

[Itsusinn]

I might to merge this PR.
But if you are going to add(or proposal) more advanced features, please consider check my another implementation of singbox-like program

https://github.com/proxy-rs/wind/

It's pretty early stage though, but structure is clear enough to read