Add External network attachment support for vcd_nsxt_edgegateway

[Didainius]

This PR adds support for adding additional External Networks (only NSX-T Segment backed) to vcd_nsxt_edgegateway. This is done using a block field external_network. It is in the same resource instead of a separate one because it uses the same API endpoint as Edge Gateway creation (a bit unfortunate, but safest way).

This feature requires VCD 10.4.1+ and uses new functionality in vmware/go-vcloud-director#610

Place in UI:

About

IP Management for Edge Gateway external network uplinks is quite simple - it doesn't support IP Spaces and only requires IP, Prefix and allocated IP count. Primary IP can either be specified manually or assigned automatically. TypeSet has enough functionality to support it therefore only one structure is introduced external_network.

A big part of the work was making sure that this works well with existing primary network uplinks and doesn't affect each other. Quite a few tests were added with the hope of ensuring interoperability.

Additional notes:

• Attempt to improve TestAccVcdNsxtDynamicSecurityGroupVdcGroupCriteriaWithVms where it sometimes fails with incorrect VM count
• Added missing changelog note for vcd/resource_vcd_vapp_network.go: Check for RESOLVED status for powering off vApp #1092
• Test config adds new field testConfig.Nsxt.NsxtImportSegment2 for improved testing

Testing

Tags used gateway nsxt network, VCD versions:

• 10.4.1
• 10.5.0
• 10.4.0
• 10.4.2

[lvirbalas]

Thanks for updates!