[pfsense_openvpn_client] Add support for "v4only" and "v6only" values for create_gw field

[samuel-emrys]

Is your feature request related to a problem? Please describe.
The only option that can currently be passed to the create_gw field is both, as per plugins/module_utils/openvpn_client.py:61:

    create_gw=dict(default='both', required=False, choices=['both']),

This populates the create_gw xml field:

<create_gw>both</create_gw>

However, pfSense also supports "IPv4 Only" (v4only) and "IPv6 Only" (v6only) values:

Describe the solution you'd like
Expansion of the allowable values of create_gw to also include v4only and v6only:

-    create_gw=dict(default='both', required=False, choices=['both']),
+    create_gw=dict(default='both', required=False, choices=['both', 'v4only', 'v6only']),

Describe alternatives you've considered
N/A

Additional context
N/A