allow using dynamic list for ipconfig instead of hardcoding the index in the key

[spettinichi]

When trying to use this provider it is difficult to create a variable amount of static IP addresses which are unknown until install time. The quantity of network interfaces can be handled variably but the accompanying static IP addresses does not work to go along with that.
I have modified this to allow dynamic IP addresses to be created while maintaining backwards compatibility for the current way of explicitly setting ipconfig0, ipconfig1, etc.

To give an example of my desired usage:

variables.tf:

variable "networks" {
  description = "defines NICs for the vm"
  type = list(object({
    model           = optional(string)
    bridge          = optional(string)
    vlan_tag        = optional(number)
    gateway         = optional(string)
    ip_address      = optional(string)
    cidr_annotation = optional(string)
  }))
  default = [{
    model           = "virtio"
    bridge          = "vmbr0"
    vlan_tag        = null
    gateway         = "10.0.0.1"
    cidr_annotation = "20"
    ip_address      = null
  }]
}

main.tf

dynamic "network" {
    for_each = var.networks
    content {
      model  = network.value.model
      bridge = network.value.bridge
      tag    = network.value.vlan_tag
    }
  }

  dynamic "ipconfig" {
    for_each = var.networks
    content {
      config = "ip=${network.value.ip_address}/${network.value.cidr_annotation},gw=${network.value.gateway}"
    }
  }

[glloyd-awetomaton]

This functionality would be great! It makes it much easier to use this resource type within a module.