netbird: add option/ auth banner

[sopex]

Important notices

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• I have read the contributing guidelines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I opened an issue first for non-trivial changes and linked it below.
• AI tools were used to create at least part of the code submitted herewith.

Describe the proposed solution

• Added option "external-ip-map"
• Added a banner to enable the plugin before trying to connect
• Added some hints

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Related issue

Closes: #5325

[fichtner]

CSVListField is encouraged as it can do per-item masks. 12.34.56.80/eth1 is pretty funky syntax -- what does it do?

[fichtner]

https://github.com/opnsense/core/blob/2b43ee5b87948347ac5a704c484001d169f059e4/src/opnsense/mvc/app/models/OPNsense/Unbound/Unbound.xml#L222-L226

[sopex]

https://docs.netbird.io/get-started/cli#flags
The whole option is written weirdly. It maps the interface to a manually-mapped IP

[fichtner]

I guess it's fine as long as we can make the regex per item instead of mocking a CSV regex

[sopex]

I am not sure how to do this differently

[sopex]

@fichtner If you have a moment to help here with the regex.

[sopex]

Except for the possible changes needed on the regex, I think it's fine.
Please do a port sync too.