Add support for UX 2.0 Service VPN

[ctvedt]

Add support for Service VPNs and Service VPN interfaces in UX 2.0 configuration groups.
(This is duplicate of #98, as I messed up that PR...)

This PR implements support for the following resources:

• service_lan_vpn_feature
• service_lan_vpn_interface_ethernet_feature

Example usage:

sdwan:
  feature_profiles:
    service_profiles:
      - name: Service_VPNS_Demo
        description: Service VPN profile
        object_trackers:
          - name: WAN_TRACKER
            id: 1
            type: Interface
            interface_name: GigabitEthernet1
        lan_vpns:
          - vpn: 10
            name: VPN_10
            ethernet_interfaces:
              - name: VPN_10_LOOPBACK
                interface_name: Loopback10
                interface_description: Management
                ipv4_address_variable: system_ip
                ipv4_subnet_mask: 255.255.255.255
                shutdown: false
              - name: VPN_10_LAN
                interface_name_variable: VPN_10_LAN_INT
                interface_description: LAN Interface for VPN 10
                ipv4_address_variable: VPN_10_LAN_IP
                ipv4_subnet_mask: 255.255.255.0
                shutdown: false
                ipv4_vrrp_groups:
                  - id: 10
                    priority_variable: VPN_10_LAN_VRRP_PRI
                    address: 10.0.0.1
                    tracking_objects:
                      - name: WAN_TRACKER
                        action: Decrement
                        decrement_value: 20
            ipv4_static_routes:
              - network_address: 0.0.0.0
                subnet_mask: 0.0.0.0
                gateway: nextHop
                next_hops:
                  - address: 10.0.0.254
                    administrative_distance: 1
              - network_address: 10.255.255.0
                subnet_mask: 255.255.255.0
                gateway: null
                null0: true

[koval4ukav]

Hello Christopher, Thank you for your work and contribution.
Cisco Team is working on increasing UX2.0 support overall in the TF module as part of SD-WAN as a code framework (here is a link to the documentation https://netascode.cisco.com/docs/data_models/sdwan/overview/). Service LAN VPN and interface release has been pending on support of other features, like route_policy (has been added recently), and fixes in the provider (CiscoDevNet/terraform-provider-sdwan#468). You can expect those features to be added in short time.