Support multiple control nets in the `StableDiffusionControlNetXSPipeline`/`StableDiffusionXLControlNetXSPipeline`

**Is your feature request related to a problem? Please describe.**
The `StableDiffusionControlNetPipeline`/`StableDiffusionXLControlNetPipeline` allow you to specify a list of control nets (i.e. depth and canny)

However, the new `ControlNetXS` pipelines only allow you provide a single control net

**Describe the solution you'd like.**
I want to be able to do:
```python
vae = AutoencoderKL.from_pretrained("madebyollin/sdxl-vae-fp16-fix", torch_dtype=torch.float16)
canny_controlnet = ControlNetXSAdapter.from_pretrained(
"UmerHA/Testing-ConrolNetXS-SDXL-canny", torch_dtype=torch.float16
)
depth_controlnet = ControlNetXSAdapter.from_pretrained(
"UmerHA/Testing-ConrolNetXS-SDXL-depth", torch_dtype=torch.float16
)
pipe = StableDiffusionXLControlNetXSPipeline.from_pretrained(
"stabilityai/stable-diffusion-xl-base-1.0", controlnet=[canny_controlnet, depth_controlnet], torch_dtype=torch.float16
)
```