Add ESG external subnet selectors support

[camrossi]

• Add external_subnet_selectors to endpoint security groups
• Support for external subnet selection in ESG configuration
• Update documentation and examples for external subnet selectors
• Include validation and proper variable definitions

[robvand]

Few remarks:

The shared bool is not idempotent:

  # module.aci.module.aci_endpoint_security_group["ts-ocp-1/openshift/egress"].aci_rest_managed.fvExternalSubnetSelector["10.61.6.80/28"] will be updated in-place
  ~ resource "aci_rest_managed" "fvExternalSubnetSelector" {
      ~ content     = {
          ~ "shared" = "yes" -> "true"
            # (1 unchanged element hidden)
        }
        id          = "uni/tn-ts-ocp-1/ap-openshift/esg-egress/extsubselector-[10.61.6.80/28]"
        # (4 unchanged attributes hidden)

Value key should probably be ip: for consistency.

[camrossi]

@robvand fixed it
However I only changed from value to ip in the ip_external_subnet_selectors. value was already used in all the other instances so I do not wanna introduce any breaking change. Perhaps a different PR should address this for ip_subnet_selectors

[robvand]

Thanks for the fixes. Seems your PR includes some changes to the set-rule module which should be omitted.