Convert index_tar to a new-style generator

[rlamy]

Though we have process_webdataset, we also need a simple generator that just returns File objects for each member of an archive similarly to the old-style UDF index_tar.

Usage should look like:

chain = DataChain.from_storage("s3://my-bucket").filter(C("file.path").glob("*.tar").gen(file=process_tar)

This would allow us to test tar file handling in datachain.lib (which we're not currently doing, as mentioned in #412), and I consider it a blocker for #438 and #367.