Add pgAdmin blueprint with Docker Compose and metadata

- Introduced pgAdmin entry in meta.json with relevant details and links.
- Added Docker Compose configuration for pgAdmin service.
- Included pgAdmin logo and template.toml for configuration settings.

blueprints/pgadmin/docker-compose.yml

@@ -0,0 +1,22 @@
+version: '3.8'
+
+services:
+  pgadmin:
+    image: dpage/pgadmin4:latest
+    restart: unless-stopped
+    environment:
+      - PGADMIN_DEFAULT_EMAIL=${PGADMIN_DEFAULT_EMAIL}
+      - PGADMIN_DEFAULT_PASSWORD=${PGADMIN_DEFAULT_PASSWORD}
+      - PGADMIN_CONFIG_SERVER_MODE=False
+      - PGADMIN_CONFIG_MASTER_PASSWORD_REQUIRED=False
+    volumes:
+      - pgadmin-data:/var/lib/pgadmin
+    networks:
+      - proxy
+
+volumes:
+  pgadmin-data:
+
+networks:
+  proxy:
+    external: true