feat: add rfc6598 reserved prefix

[DuckSoft]

Just have been using Outline Shadowsocks Server for a great long while, and it worked pretty fine though,
utill few days ago an abuse alert Email from Hetnzer was received, warning that my server is "sending malicious UDP attack traffic".

And here's their "evidence" -- UDP access log from their side:

To them it seemed that my server is constantly attacking that 100.64.blah.blah address via UDP flooding.

A quick investigation revealed that this IP address is actually an IP address that is used and allocated by one of my users' ISP, and somehow the traffic is not correctly discarded. Looking at the code of Outline Shadowsocks Server, there is no 100.64.0.0/10 CGNAT prefix in private addresses.

So I've made this Pull Request. Hopefully this will prevent more users under CGNAT receiving some sorts of "Abuse Alert" from any VPS Providers.

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[DuckSoft]

@googlebot I signed it!

[alalamav]

LGTM, thank you for the patch.

[fortuna]

This is now released as outline-ss-server v1.3.5: https://github.com/Jigsaw-Code/outline-ss-server/releases/tag/v1.3.5
We still need to release the change to outline-server.