Adds peering example

[amandakarina]

Hey folks,

This pull request adds peering example for step 4-projects.

Could you please, take a look?

Thanks!

[rjerrems]

FYI it looks like tests are failing due to peering occurring concurrently ``` Error: Error adding network peering: googleapi: Error 400: There is a peering operation in progress on the local or peer network. Try again later., badRequest

     on .terraform/modules/projects_bu1_dev.peering/modules/network-peering/main.tf line 22, in resource "google_compute_network_peering" "local_network_peering":
     22: resource "google_compute_network_peering" "local_network_peering" {
   
   
   
   Error: Error adding network peering: googleapi: Error 400: There is a peering operation in progress on the local or peer network. Try again later., badRequest
   
     on .terraform/modules/projects_bu1_prod.peering/modules/network-peering/main.tf line 22, in resource "google_compute_network_peering" "local_network_peering":
     22: resource "google_compute_network_peering" "local_network_peering" {```

[bharathkkb]

LGTM and thanks for adding the tests @amandakarina
/cc @rjerrems

[rjerrems]

LGTM