FileField: uri passthrough

[u8sand]

Some URIs like s3 would be better off not having to be copied onto the system; in fact, this also has implications for standard uploads which go to s3 as well.

It would probably be opt in but essentially, FileField could have a new option mode with the following possibilities:

• mode='copy' (current): copy the file into the appyter local directory for execution
• mode='mount': mount the file from its URI (e.g s3) into the local directory during execution
• mode='auto' (new default): mount if s3 else copy & mount from shared input
• mode='uri': pass through the exact uri, up to the notebook to handle it, i.e. for pandas

We also would need to update the UI to permit specifying a URI instead of uploading a file.