Supports directory mounts for model types provided by extensions

[KagurazakaNyaa]

Some extensions will create other model directories under the model directory, such as Deforum(#255) and dreambooth(https://github.com/d8ahazard/sd_dreambooth_extension/blob/main/dreambooth/shared.py#L306)
In order to support the persistence of these models, we have to manually mount them in data/config/auto/startup.sh, and it is easy to miss the model types of newly installed extensions in the future.
So why don't we just persist the entire models directory to /data/models to solve this problem?

like this
https://github.com/KagurazakaNyaa/stable-diffusion-webui-docker/commit/5d9f3daa1ff48685ac8b6e6823f8921d5607f482

[AbdBarho]

This works elegantly for auto, but maybe not as elegant for the others.

[KagurazakaNyaa]

Maybe we can make it compatible with other types of webui by creating a directory alias under data/models.
But I doubt that anyone will use multiple different webuis in the same directory at the same time.
Maybe my attempt in this commit can be referred to to solve this problem.

https://github.com/KagurazakaNyaa/stable-diffusion-webui-docker/commit/d1e5d5905902b4413efc051edae2d8388d573170