Add initial (trivial) pubsub module and faas module that supports pubsub

Relates #3 (closed)

Follow up MRs or split tickets for alerting and working dead letter queue

terraform/modules/faas/main.tf

+resource "google_project_service_identity" "pubsub_sa" {
+  provider = google-beta
+  project  = var.project_id
+  service  = "pubsub.googleapis.com"
+}
+
+resource "google_cloud_run_service" "faas" {
+  name     = var.name
+  location = var.location
+
+  template {
+    metadata {
+      annotations = {
+        "run.googleapis.com/service-account" = "serviceAccount:${google_project_service_identity.pubsub_sa.email}"
+      }
+    }
+
+    spec {
+      containers {
+        image = var.container_image
+      }
+    }
+  }
+
+  traffic {
+    percent         = 100
+    latest_revision = true
+  }
+}
+
+resource "google_eventarc_trigger" "trigger" {
+  name     = "${var.name}-trigger"
+  location = var.location
+
+  transport {
+    pubsub {
+      topic = var.topic_id
+    }
+  }
+
+  destination {
+    cloud_run_service {
+      service = google_cloud_run_service.faas.name
+    }
+  }
+
+  matching_criteria {
+    attribute = "type"
+    value     = "google.cloud.pubsub.topic.v1.messagePublished"
+  }
+
+  service_account = "serviceAccount:${google_project_service_identity.pubsub_sa.email}"
+}