add snat gateway support for lvs_v2

[jlijian3]

Add snat gateway feature like iptales snat，compatible with VS/NAT VS/FULLNAT，you can config them together，like below：
virtual_server 1.1.3.100 80 {
...
}

fwmark 1 only meam it's snat-gateway service，not depend on iptables.

virtual_server fwmark 1 {
snat_rule {
from 192.168.40.0/24 #source address
oif eth1
snat_ip 1.1.2.100-1.1.2.200
algo sdh #sdh sh random
}
}

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1 out of 2 committers have signed the CLA.

✅ jlijian3
❌ monkeyerKong
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[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ jlijian3
❌ monkeyerKong
_{You have signed the CLA already but the status is still pending? Let us recheck it.}