Improve validation error message

[chrismarget-j]

│ Error: Invalid Attribute Combination
│ 
│   with apstra_datacenter_connectivity_template_interface.example,
│   on main.tf line 45, in resource "apstra_datacenter_connectivity_template_interface" "example":
│   45: resource "apstra_datacenter_connectivity_template_interface" "example" {
│ 
│ At least one attribute out of
│ [ip_links["foo"].bgp_peering_ip_endpoints["bar"].ipv4_address,ip_links["foo"].bgp_peering_ip_endpoints["bar"].ipv4_address.<.ipv6_address]
│ must be specified

This bit is clunky: ipv4_address.<.ipv6_address