[DCP Ingestion - Stale Reads] Enable Stale Reads in CDC Web server which runs mixer (#31)

* Pass in use stale reads to environment variables for CDC webservice which starts up mixer.

* Only use stale reads if spanner is enabled.

* Move stale reads to CDC only.

* Adds GCP PRoject ID which will go in the spanner config.

* Removes the USE_STALE_READS flag which is no longer needed

Author: gmechali

infra/dcp/modules/cdc/main.tf

@@ -262,6 +262,11 @@ resource "google_cloud_run_v2_service" "dc_web_service" {
         }
       }
 
+      env {
+        name  = "GCP_PROJECT_ID"
+        value = var.project_id
+      }
+
       dynamic "env" {
         for_each = local.cloud_run_shared_env_variable_secrets
         content {