add logic to construct namespace from provided namespace and plugin id

Signed-off-by: Shijun Wang <shijun@unity3d.com>

docs/backend-system/core-services/cache.md

@@ -7,6 +7,39 @@ description: Documentation for the Cache service
 
 This service lets your plugin interact with a cache. It is bound to your plugin too, so that you will only set and get values in your plugin's private namespace.
 
+## Configuration
+
+The cache service can be configured using the `backend.cache` section in your `app-config.yaml`:
+
+```yaml
+backend:
+  cache:
+    store: redis # or 'valkey', 'memcache', 'memory'
+    connection: redis://localhost:6379
+
+    # Store-specific configuration (Redis/Valkey only)
+    redis:
+      client:
+        # Optional: Global namespace prefix for all cache keys
+        namespace: 'my-app'
+        # Optional: Separator used between namespace and plugin ID (default: ':')
+        keyPrefixSeparator: ':'
+        # Other Redis-specific options...
+        clearBatchSize: 1000
+        useUnlink: false
+```
+
+### Namespace Configuration
+
+For Redis and Valkey stores, you can configure a global namespace that will be prefixed to all cache keys:
+
+- **Without namespace**: Cache keys use only the plugin ID (e.g., `catalog:some-key`)
+- **With namespace**: Cache keys use the format `namespace:pluginId:key` (e.g., `my-app:catalog:some-key`)
+
+The `keyPrefixSeparator` controls what character is used between the namespace and plugin ID (defaults to `:`).
+
+**Note**: Memory and Memcache stores do not support namespace configuration and will always use the plugin ID directly.
+
 ## Using the service
 
 The following example shows how to get a cache client in your `example` backend plugin and setting and getting values from the cache.