Include 100.64.0.0/10 in ALLOWED_LAN_NETS

[chaeynz]

I added the 100.64.0.0/10 address space to ALLOWED_LAN_NETS.

100.64.0.0/10 is reserved by IANA as Shared Address Space, is filtered on the public Internet through routing filters (May not be advertised) and can therefore be treated as local.
https://datatracker.ietf.org/doc/html/rfc6598

It mainly serves ISPs, but can also be used on the internal network by organizations of any kind.

Please merge my PR, so that users will be able to access parts of their network where this address space is used, while still being connected to the Mullvad VPN.

Closes #6086

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This change is 

[0xBA5E64]

Really hoping to see this get merged! Mullvad's lack of interoperability with Tailscale is particularly frustrating since the two projects do already have interoperability though Mullvad exit nodes on Tailscale.

However, as an ordinary Mullvad user however, I really hope to be able to use something like this instead.

[faern]

I'm not comfortable just calling this network a "LAN" and blindly allow it for everyone who wants local network traffic. This network is primarily intended for CGN. This would be a risk to most customers, and a benefit to probably very few.

If you run Linux, you can apply custom Firewall rules and allow 100.64.0.0/10 by yourself. See this guide: https://mullvad.net/en/help/split-tunneling-with-linux-advanced

Maybe we should consider adding a feature where the user can enter a list of networks that they want unblocked?