Use placeholder revision in urls in cached responses
Previously, we were using the dataset revision in the URLs of image/audio files of cached responses of /first-rows.
However when a dataset gets its README updated, we update the `dataset_git_revision` of the cache entries and the location of the image/audio files on S3 but we don't modify the revision in the URLs in the cached response.
This resulted in the Viewer not showing the images after modifying the readme of a dataset.

<img width="1020" alt="image" src="https://github.com/huggingface/dataset-viewer/assets/42851186/6242d9d6-bc2e-42b1-9edd-9f01173b6cd8">


I fixed that for future datasets by not using the revision in the URLs anymore and use a placeholder that is replaced by `dataset_git_revision` when the cached response is accessed

## Implementation details

I modified the URL Signer logic to also insert the revision in the URL and renamed it to a URL Preparator. It takes care of inserting the revision and signing the URLs.

close https://github.com/huggingface/dataset-viewer/issues/2965
Use placeholder revision in urls in cached responses: Previously, we were using the dataset revision in the URLs of image/audio files of cached responses of /first-rows.
However when a dataset gets its README updated, we update the `dataset_git_revision` of the cache entries and the location of the image/audio files on S3 but we don't modify the revision in the URLs in the cached response.
This resulted in the Viewer not showing the images after modifying the readme of a dataset.

<img width="1020" alt="image" src="https://github.com/huggingface/dataset-viewer/assets/42851186/6242d9d6-bc2e-42b1-9edd-9f01173b6cd8">


I fixed that for future datasets by not using the revision in the URLs anymore and use a placeholder that is replaced by `dataset_git_revision` when the cached response is accessed

## Implementation details

I modified the URL Signer logic to also insert the revision in the URL and renamed it to a URL Preparator. It takes care of inserting the revision and signing the URLs.

close https://github.com/huggingface/dataset-viewer/issues/2965