VPN: IPsec: Advanced Settings - move to MVC and add some options

[AdSchellevis]

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

In some cases we would like to be able to customize settings in strongswan.conf (https://docs.strongswan.org/docs/5.9/config/strongswanConf.html).

Describe the solution you like

Since we already have a legacy advanced settings page, we should migrate those and optionally add new settings.
Currently on the wishlist are settings to change retransmit behavior:

charon {
    retransmit_tries = xxx
    retransmit_timeout = xx
    retransmit_base = xx
}

(https://docs.strongswan.org/docs/5.9/config/retransmission.html)

Describe alternatives you considered

none

Additional context