fix: Router custom-learned-route-priority undefined behavior

[modular-magician]

Bug: b/378841773
Issue: custom-learned-route-priority is an optional field .
However, TF would set custom-learned-route-priority to be 0 even if the user has not explicitly set the value to be 0 when there is an update to the resource.

Repro steps:

Create a Router peer resource without custom-learned-route-priority value set.

resource "google_compute_router_peer" "peer" {
  name                      = "tf-test-my-router-peer-1"
  router                    = google_compute_router.router.name
  region                    = google_compute_router.router.region
  interface                 = "interface-1"
  peer_asn                  = 65513
  advertise_mode            = "CUSTOM"
}

Query gcloud to check the value of custom-learned-route-priority, it will be empty
gcloud compute routers describe {router-name}

Update the router peer resource (ex: advertised_route_priority = 100) without acustom-learned-route-priority. However, TF would add custom-learned-route-priority = 0 in the update api call.

resource "google_compute_router_peer" "peer" {
  name                      = "tf-test-my-router-peer-1"
  router                    = google_compute_router.router.name
  region                    = google_compute_router.router.region
  interface                 = "interface-1"
  peer_asn                  = 65513
  advertised_route_priority = 100
  advertise_mode            = "CUSTOM"
}

Query gcloud to check the value of custom-learned-route-priority, it will be 0 even though it is not set by the user.
gcloud compute routers describe {router-name}

Release Note Template for Downstream PRs (will be copied)

compute: fixed a issue where `custom_learned_route_priority` was accidentally set to 0 during updates in 'google_compute_router_peer'

bug: added support for setting `custom_learned_route_priority` to 0 in 'google_compute_router_peer' by adding the `zero_custom_learned_route_priority` field

Derived from GoogleCloudPlatform/magic-modules#12355