OpenVPN: Add possibility to use custom tls-verify script

[mkrauser]

Hey there,

first of all, thank for this great software. I really enjoy OPNSense.

We are currently using a VPN-Server and a custom tls-verify-script. It seems, it is currently not possible to customise or extend the behaviour of the internal tls-verify script.

This would be gread for us.

Would you consider accepting a PR for this?

[OPNsense-bot]

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