[ModelLoader][Feature] Add rfork support for fast model loading (#7392)

### What this PR does / why we need it?
Support an new load format: RFORK

For implementation details of this feature, please refer to #7441


### Does this PR introduce _any_ user-facing change?

add an new options for load-format: rfork

e.g.
```bash
vllm serve /workspace/models/Qwen3-8B --load-format rfork
```

### How was this patch tested?

- vLLM version: v0.17.0
- vLLM main:
4034c3d32e

Signed-off-by: Marck <1412354149@qq.com>

docs/source/user_guide/feature_guide/index.md

@@ -13,6 +13,7 @@ structured_output
 lora
 eplb_swift_balancer
 netloader
+rfork
 Multi_Token_Prediction
 dynamic_batch
 epd_disaggregation