Allow to protect not yet protected .ovpn private key with user account's password

### Problem

I created a .ovpn file where the included private key is not password protected.
I want the private key to be password protected at rest.

### Proposal

When importing an .ovpn file that contains a non-password-protected private key, show a dialog which allows the user to opt in for protecting the private key with the user accounts password.
It should behave like a password protected SSH key that you can decrypt by providing your user account's password (there is a way to have your SSH key automatically be unlocked when you log in to your account - this should also still be possible for .ovpn).
I don't know where all the things are stored and how the user account login auto-unlock thingy works so I don't know how to describe the above in a technically correct way.

Importing of ovpn via opening the ovpn file (#354) should also allow to choose to protect the private key in the imported file

### Prior Art (Optional)

_No response_