Add support for subnet-level service network user grants to project module, improve docs (#1907)

* improve project factory example

* light refactor of project modules shared vpc internals and docs

* add support for subnet-level grants on host project

tests/modules/project/examples/shared-vpc-subnet-grants.yaml

@@ -0,0 +1,66 @@
+# Copyright 2023 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+values:
+  module.host-project.google_compute_shared_vpc_host_project.shared_vpc_host[0]:
+    project: test-host
+    timeouts: null
+  module.host-project.google_project.project[0]:
+    auto_create_network: false
+    billing_account: 123456-123456-123456
+    folder_id: '1122334455'
+    labels: null
+    name: test-host
+    org_id: null
+    project_id: test-host
+    skip_delete: false
+    timeouts: null
+  module.service-project.google_compute_shared_vpc_service_project.shared_vpc_service[0]:
+    deletion_policy: null
+    host_project: test-host
+    service_project: test-service
+    timeouts: null
+  module.service-project.google_compute_subnetwork_iam_member.shared_vpc_host_robots["europe-west1:gce:compute"]:
+    condition: []
+    project: test-host
+    region: europe-west1
+    role: roles/compute.networkUser
+    subnetwork: gce
+  module.service-project.google_project.project[0]:
+    auto_create_network: false
+    billing_account: 123456-123456-123456
+    folder_id: '1122334455'
+    labels: null
+    name: test-service
+    org_id: null
+    project_id: test-service
+    skip_delete: false
+    timeouts: null
+  module.service-project.google_project_service.project_services["compute.googleapis.com"]:
+    disable_dependent_services: false
+    disable_on_destroy: false
+    project: test-service
+    service: compute.googleapis.com
+    timeouts: null
+
+counts:
+  google_compute_shared_vpc_host_project: 1
+  google_compute_shared_vpc_service_project: 1
+  google_compute_subnetwork_iam_member: 1
+  google_project: 2
+  google_project_service: 1
+  modules: 2
+  resources: 6
+
+outputs: {}