URL validation/access fixes (#4695)
* is_valid_url: share a requests session for performance
* Rename is_valid_url to probe_url (because that makes sense for what it does); add is_http_url_like
* Do not allow encode_url_to_base64 to encode error responses
* Do not allow download_tmp_copy_of_file to download error responses as valid files
* Changelog
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Co-authored-by: Abubakar Abid <abubakar@huggingface.co>
