add model router plugin (#1414)

plugins/wasm-cpp/extensions/model_router/README_EN.md

@@ -0,0 +1,63 @@
+## Function Description
+The `model-router` plugin implements the functionality of routing based on the `model` parameter in the LLM protocol.
+
+## Runtime Properties
+
+Plugin Execution Phase: `Default Phase`
+Plugin Execution Priority: `260`
+
+## Configuration Fields
+
+| Name               | Data Type   | Filling Requirement | Default Value         | Description                                           |
+| -------------------- | ------------- | --------------------- | ---------------------- | ----------------------------------------------------- |
+| `enable`            | bool        | Optional             | false                 | Whether to enable routing based on the `model` parameter |
+| `model_key`         | string      | Optional             | model                 | The location of the `model` parameter in the request body |
+| `add_header_key`    | string      | Optional             | x-higress-llm-provider | The header where the parsed provider name from the `model` parameter will be placed |
+
+## Effect Description
+
+To enable routing based on the `model` parameter, use the following configuration:
+
+```yaml
+enable: true
+```
+
+After enabling, the plugin extracts the provider part (if any) from the `model` parameter in the request, and sets it in the `x-higress-llm-provider` request header for subsequent routing. It also rewrites the `model` parameter to the model name part. For example, the original LLM request body is:
+
+```json
+{
+    "model": "openai/gpt-4o",
+    "frequency_penalty": 0,
+    "max_tokens": 800,
+    "stream": false,
+    "messages": [{
+        "role": "user",
+        "content": "What is the GitHub address for the main repository of the Higress project?"
+    }],
+    "presence_penalty": 0,
+    "temperature": 0.7,
+    "top_p": 0.95
+}
+```
+
+After processing by the plugin, the following request header (which can be used for routing matching) will be added:
+
+`x-higress-llm-provider: openai`
+
+The original LLM request body will be modified to:
+
+```json
+{
+    "model": "gpt-4o",
+    "frequency_penalty": 0,
+    "max_tokens": 800,
+    "stream": false,
+    "messages": [{
+        "role": "user",
+        "content": "What is the GitHub address for the main repository of the Higress project?"
+    }],
+    "presence_penalty": 0,
+    "temperature": 0.7,
+    "top_p": 0.95
+}
+```