Add table transformer [v2]
# What does this PR do?

This PR adds [Table Transformer](https://github.com/microsoft/table-transformer) by Microsoft, as a separate model, rather than tweaking the existing DETR implementation.
@sgugger I've removed the non-Table Transformer related stuff to #19644 ;)<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger PR is ready, feel free to approve :)