Add segmentation backbones

[hermancollin]

We need to add more segmentation backbones to transform this into a real benchmark; this is important for reviewers. A few options:

• scaling nnUNet ResEnc (M/L/XL)
• MedNeXt
• transformer-based (e.g. SwinUNetR)
• mamba-based (e.g. U-Mamba)