Skip identical files in `push_to_hub` instead of overwriting
_The documentation is not available anymore as the PR was closed or merged._
Skip identical files instead of overwriting them to save bandwidth and circumvent (user-side/server-side) errors, which can arise when working with large datasets due to long-lasting HTTP connections, by repeating calls to `push_to_hub` to resume an upload.

To be able to check if an upload can be resumed, this PR modifies the shard naming scheme from:
```
data/{split}-[0-9][0-9][0-9][0-9][0-9]-of-[0-9][0-9][0-9][0-9][0-9].parquet
```
to:
```
data/{split}-[0-9][0-9][0-9][0-9][0-9]-of-[0-9][0-9][0-9][0-9][0-9]-<SHARD_FINGERPRINT>.parquet
```
cc @LysandreJik 