ipsec: move legacy implementation into a plugin

[fichtner]

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Is your feature request related to a problem? Please describe.

Is your feature request related to a problem? Please describe.

IPsec tunnel GUI has been deprecated for a while.

Describe the solution you like

To keep the code and user base we will move it to a plugin. os-ipsec-legacy is probably fitting leaving the door open for os-ipsec.

The OpenVPN server wizard will not be part of the plugin as the wizard is also going to be replaced in 25.7.

Describe alternatives you considered

Removing the code.

Additional context

N/A