Sign cached assets urls
See #2280 

when the storage is S3:
- [x] add a key pair to the configuration to be able to forge signed URLs for the assets bucket (we use the private key to sign the URL, we pass the public key as a query parameter in the URL) -> done here (internal): https://github.com/huggingface/infra/pull/796
- [x] add a method to generate a signed URL for <strike>a S3 object</strike> an asset URL (see https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/private-content-creating-signed-url-canned-policy.html -> it's a matter of creating the expiration date, creating the signature, and appending these parameters as query parameters of the URL)
- [x] test s3 + cloudfront (the infra team prepared an environment)
- [x] for /rows, /filter, /search: return signed URLs

Sign cached assets urls: See #2280 

when the storage is S3:
- [x] add a key pair to the configuration to be able to forge signed URLs for the assets bucket (we use the private key to sign the URL, we pass the public key as a query parameter in the URL) -> done here (internal): https://github.com/huggingface/infra/pull/796
- [x] add a method to generate a signed URL for <strike>a S3 object</strike> an asset URL (see https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/private-content-creating-signed-url-canned-policy.html -> it's a matter of creating the expiration date, creating the signature, and appending these parameters as query parameters of the URL)
- [x] test s3 + cloudfront (the infra team prepared an environment)
- [x] for /rows, /filter, /search: return signed URLs
