feat: Add Google Drive storage backend with custom endpoint support (#613)

* feat: Add Google Drive storage backend with custom endpoint support

- Implement Google Drive storage backend using service account authentication
- Add support for custom Google Drive API endpoints for testing
- Include comprehensive test setup with mock services
- Add configuration options for Google Drive credentials and folder ID
- Update documentation with Google Drive configuration examples

* remove debug messages

* Improve tests

* Add mounting tip on documentation

* Replace deprecated lib usage

* Fix identation

* Fix googledrive tests

* Fix googledrive support for _FILE credentials

* Remove googledrive test unecessary echos

test/googledrive/docker-compose.yml

@@ -0,0 +1,52 @@
+services:
+  openapi_mock:
+    image: muonsoft/openapi-mock:0.3.9
+    environment:
+      OPENAPI_MOCK_USE_EXAMPLES: if_present
+      OPENAPI_MOCK_SPECIFICATION_URL: '/etc/openapi/googledrive_v3.yaml'
+    ports:
+      - 8080:8080
+    volumes:
+      - ${SPEC_FILE:-./googledrive_v3.yaml}:/etc/openapi/googledrive_v3.yaml
+
+  oauth2_mock:
+    image: ghcr.io/navikt/mock-oauth2-server:1.0.0
+    ports:
+      - 8090:8090
+    environment:
+      PORT: 8090
+      JSON_CONFIG_PATH: '/etc/oauth2/config.json'
+    volumes:
+      - ./oauth2_config.json:/etc/oauth2/config.json
+
+  backup:
+    image: offen/docker-volume-backup:${TEST_VERSION:-canary}
+    hostname: hostnametoken
+    depends_on:
+      - openapi_mock
+      - oauth2_mock
+    restart: always
+    environment:
+      BACKUP_FILENAME_EXPAND: 'true'
+      BACKUP_FILENAME: test-$$HOSTNAME.tar.gz
+      BACKUP_CRON_EXPRESSION: 0 0 5 31 2 ?
+      BACKUP_RETENTION_DAYS: ${BACKUP_RETENTION_DAYS:-7}
+      BACKUP_PRUNING_LEEWAY: 5s
+      BACKUP_PRUNING_PREFIX: test
+      GOOGLE_DRIVE_ENDPOINT: http://openapi_mock:8080
+      GOOGLE_DRIVE_TOKEN_URL: http://oauth2_mock:8090/issuer1/token
+      GOOGLE_DRIVE_CREDENTIALS_JSON_FILE: /etc/gdrive/credentials.json
+      GOOGLE_DRIVE_FOLDER_ID: "root"
+    volumes:
+      - app_data:/backup/app_data:ro
+      - ./credentials.json:/etc/gdrive/credentials.json
+
+  offen:
+    image: offen/offen:latest
+    labels:
+      - docker-volume-backup.stop-during-backup=true
+    volumes:
+      - app_data:/var/opt/offen
+
+volumes:
+  app_data: