DFW rule management resource vcd_nsxt_distributed_firewall_rule

[Didainius]

Closes #970

This PR introduces a new resource and data source vcd_nsxt_distributed_firewall_rule to manage Distributed firewall rules one by one. We already have a resource vcd_nsxt_distributed_firewall, but that one manages all rules at once and there is a request #970 to manage rules one by one.

Notes

• There is no API to create a single distributed firewall rule, so this functionality is handled in Go SDK for VCD VdcGroup.CreateDistributedFirewallRule. As a consequence - firewall rules will not be created in parallel, but instead one by one (as the resource will have locks).
• There is an above_rule_id field in the new resource to handle the firewall rule position during create operation.
• By default, a distributed firewall gets one default rule created after it is activated in resource vcd_vdc_group. This would make it very inconvenient to work with firewall rules using new resources (although possible by using data source and then field above_rule_id to put all firewall rules above). To make things more convenient, vcd_vdc_group resource has a new flag remove_default_firewall_rule that would remove the default firewall rule.

Note for reviewers - the "internals" of resource are very similar to vcd_nsxt_distributed_firewall. There difference is here we don't have element nesting in a TypeSet

Patch for API V38.0

• vcd_nsxt_app_port_profile has a patch for similar diff errors in commit 38c907b:

~ resource "vcd_nsxt_app_port_profile" "custom-org1" {
   id          = "urn:vcloud:applicationPortProfile:fbcf286a-4a33-40c7-8312-c5a71449a54f"
   name        = "custom_app_prof"
   # (4 unchanged attributes hidden)

 - app_port {
     - port     = [
         - "any",
       ] -> null
     - protocol = "ICMPv4" -> null
   }
 + app_port {
     + port     = []
     + protocol = "ICMPv4"
   }

[lvirbalas]

It is a magical feature! I have provided some "fresh eyes" thoughts.

[lvirbalas]

This looks great! The last open item I have is on naming #1076 (comment) , but it will depend on input from a wider audience.

[adezxc]

Looks good to me, great PR!

[adambarreiro]

LGTM