Add External Gateway Support for Network Service (Neutron) #3567

What is missing?

I’d like to request adding support in Gophercloud for managing external gateways on routers within the Network (Neutron) service.

OpenStack API reference:
“Add External Gateways to Router” — https://docs.openstack.org/api-ref/network/v2/index.html#add-external-gateways-to-router
“Update External Gateways of Router” — https://docs.openstack.org/api-ref/network/v2/index.html#update-external-gateways-of-router
“Remove External Gateways from Router” — https://docs.openstack.org/api-ref/network/v2/index.html#remove-external-gateways-from-router