BGP Route Policy Support

[vijaytdh]

TL;DR

Support configuring BGP policies for google_compute_router_peer

Terraform Resources

https://cloud.google.com/network-connectivity/docs/router/concepts/bgp-route-policies-overview

https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_router_peer#example-usage---router-peer-export-and-import-policies

Detailed design

We would like to associate BGP policies with the `google_compute_router_peer` resource - the `import_policies` and `export_policies` arguments are list of just the name of the policy. 

Note this is only available in the beta provided as far as I know and I am not sure if this repo allows the use of beta provider resources/features.

It would be good if the module were to make the `import_policies` and `export_policies` arguments configurable. Note, I am *not* asking for the policies to be created by this module.

e.g. I am looking for the `tunnels` variable to be updated to 


variable "tunnels" {
  description = "VPN tunnel configurations, bgp_peer_options is usually null."
  type = map(object({
    bgp_peer = object({
      address = string
      asn     = number
    })
    bgp_session_name = optional(string)
    bgp_peer_options = optional(object({
      ip_address          = optional(string)
      advertise_groups    = optional(list(string))
      advertise_ip_ranges = optional(map(string))
      advertise_mode      = optional(string)
      route_priority      = optional(number)
      import_policies     = optional(list(string))
      export_policies     = optional(list(string))
    }))
    bgp_session_range               = optional(string)
    ike_version                     = optional(number)
    vpn_gateway_interface           = optional(number)
    peer_external_gateway_self_link = optional(string, null)
    peer_external_gateway_interface = optional(number)
    shared_secret                   = optional(string, "")
  }))
  default = {}
}


So something like the following can be done in the `main.tf`

```hcl
  import_policies = each.value.bgp_peer_options.import_policies == null ? null : each.value.bgp_peer_options.import_policies
  export_policies = each.value.bgp_peer_options.export_policies == null ? null : each.value.bgp_peer_options.export_policies

### Additional information

_No response_