[MaskFormer] Add support for ResNet backbone
# What does this PR do?

This PR is part 3 of 3 of the big #20204 PR.

This PR does 2 things:
1) it makes sure that ResNet is supported as backbone for MaskFormer, besides Swin. It leverages the `AutoBackbone` class for this.
2) <s> it makes sure that MaskFormer defaults to Swin as backbone, not relying on MaskFormerSwin, but just on plain Swin. For this, the argument `output_hidden_states_before_downsampling` is added to `SwinConfig`. </s> => `SwinBackbone` will be added in a separate PR

To do:

- [x] convert all remaining MaskFormer checkpoints
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger feel free to approve, I'm currently uploading all checkpoints to the hub :)