Support different ipv6 network device in network-tunnel-manager.sh

[heri16]

On many VPS providers, the IPv4 uplink network interface is eth0, while the IPv6 uplink may be on a different interface (e.g., eth1 or another device).

This PR:

• Improves automatic detection of the uplink network interface for both IPv4 and IPv6.
• Adds support for configurations where the IPv4 and IPv6 uplinks are on different interfaces, while remaining fully compatible with setups where they share the same interface.
• Changes the expect environment variable name from NETWORK_DEVICE to IPV4_UPLINK_DEV and IPV6_UPLINK_DEV

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This change is 

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Thank you for making this first PR

[serinko]

Hey, it looks good. Thank you for your PR.
You haven't ticked the env var but I see it added in the PR

Please redirect it from develop to feat/operators/ntm-upgrades.

This var is used here:
https://github.com/nymtech/nym/blob/develop/scripts/nym-node-setup/nym-node-cli.py#L578
https://github.com/nymtech/nym/blob/develop/scripts/nym-node-setup/nym-node-cli.py#L643

If you want you can toggle that too, otherwise I will fix that one

[heri16]

For the record:
"ip -6 -o route show default" was not reliable.
On my VPS machine (Debian 13), it requires "print $3" while the original script uses "print $5".
See:

nonroot@localhost:~$ ip -6 -o route show default
default dev eth1.1 proto static metric 1024 pref medium

[heri16]

https://github.com/nymtech/nym/blob/develop/scripts/nym-node-setup/nym-node-cli.py#L643

Should we have:

• Option A: --uplink-dev-v4 and --uplink-dev-v6
• Option B: --uplink-dev and --uplink-dev-v6 where v6 defaults to autodetection if unspecified?

@serinko

Also, I've found another issue with the quic_bridge_deployment.sh on machines such as mine.
Would you prefer a different PR? This line containing UPLINK_DEV is used by quic_bridge_deployment.sh: https://github.com/nymtech/nym/blob/develop/scripts/nym-node-setup/nym-node-cli.py#L577

[heri16]

Please review latest changes to this PR.

[heri16]

[serinko]

I would prefer this one as the program should ask minimum from the operator

* Option B: `--uplink-dev` and `--uplink-dev-v6` where v6 defaults to autodetection if unspecified?

If that issue on QUIC connects to this interface, feel free to add it to this PR.

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