Add ExtractAttachmentsPreprocessor

[tuncbkose]

Adds a new preprocessor to extract notebook attachments into individual files, so that conversion outputs (like .md/.tex files) work properly.

Fixes #1977.

Todos

• Finish implementation
• Finish preprocessor tests
• Add tests related to FilesWriter tweaks

[tuncbkose]

The previous iteration was fine with the tests, but it looks like the way I associated attachment directories with specific notebooks broke something. Will look into it soon.

[tuncbkose]

I am not sure what is the best way to enable this preprocessor. I initially thought of adding it to TemplateExporter.default_config but ran into #1980.

For now, I enabled it for markdown and latex exporters.

[tuncbkose]

It might be worth noting that I am aware ExtractOutput is pretty similar, but I am under the impression that cell outputs and attachments are different enough mechanisms to warrant different preprocessors. Still, they should probably work relatively similarly, for example if a directory is created to save related files.

I'll take a look to see how it works and make appropriate changes later.

[tuncbkose]

Updated with latest commits to main

[blink1073]

Thank you!