feat: add option to disable management datadir mount

[moh2a]

In airgap environments, management fails if it cannot download the geolocation database.
To support airgap setups, this change allows the /var/lib/netbird volume mount to be
conditionally disabled via a Helm value. This lets users prepopulate the geolocation
files directly in the management image without the mount overwriting them.

Closes #19

[moh2a]

Hi @mohamed-essam

I’ve implemented a change to make the /var/lib/netbird volume mount optional to support airgap environments (see issue #19).

Could you please take a look when you have a moment? I’d really appreciate your feedback or suggestions.

Thank you! 🙏