[UX2.0][20.15] add support for etherchannel

Support for configuring EtherChannel was added to configuration groups in 20.15.1, with support for both transport and service side etherchannels. Would like to see this implemented for the provider as well!

Resources:
- `sdwan_transport_wan_vpn_interface_ethernet_feature`
- `sdwan_service_lan_vpn_interface_ethernet_feature`
