Allow setting OpenVPN client credentials

Add a way to set/update/delete OpenVPN client credentials from reForis.

UI

Based on the discussion, I would propose something similar to how we are handling DHCP static leases. I.e. something like following wireframes:

Click to expand

With following modal:

The same modal could be also used for adding new VPN client configuration, but I am not sure how to make the UI of "upload config and optionally set the client credentials".

Feel free to change the design to be aligned with reForis style ;-)

JSON messages

Changes in request sent from reforis.

Adding new client

Before

{
    "config": {"type": "string"}
}

New

{
    "config": {"type": "string"},
    "credentials": { <-- this is optional attribute
        "username": {"type": "string"},
        "password": {"type": "string"}
    }
}

Set/Update client

Before

{
    "id": {"$ref": "#/definitions/client_id"},
    "enabled": {"type": "boolean"}
}

New

{
    "id": {"$ref": "#/definitions/client_id"},
    "enabled": "type": "boolean"},
    "credentials": { <-- this is optional attribute
        "username": {"type": "string"},
        "password": {"type": "string"}
    }
}

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Depends on: turris/foris-controller/foris-controller-openvpn_client-module#5 (closed)