OpenVPN tap bridge configurations without a tunnel network need a route-gateway for routes/redirects
When using a bridged tap configuration, routing through tap to the Internet or other subnets outside of the bridged interface requires a gateway, but with a bridged tap setup the server can't give the client a gateway by default because the tunnel network must be empty.
In these cases, the server must manually push a gateway to clients using advanced options (e.g.
push "route-gateway 192.168.1.1";)
The user must already pick an interface to bridge, so adding one more option to trigger the route-gateway push based on that can get the job done.
Unfortunately, OpenVPN only supports this for IPv4, there is no working route-ipv6-gateway that can be pushed at this time.
If a user needs a more advanced solution (e.g. using a route-gateway of a VIP or some other address) then they can continue to use advanced options to set it manually.
This is similar to #5835 but different enough to require a separate solution.