added the download to xslx feature + button

[alicecommits]

Hi,
I wanted to add the capability for the button to write the wrangled dataframe into .xlsx format.
For this, I added a parameter target_format that accepts txt, csv, or xlsx, and download_button function processes the csv accordingly.
For now, I haven't handled the case where the user does not specify the format, if it has to materialize into an error or select a certain default format option (like csv).
Using BytesIO lib, I stored the processed data in an _io.BytesIO object, then converted to bytes object, following this stack overflow thread

[alicecommits]

Note: my PR also contains the modif label=" " and label_visibility='collapsed' , otherwise my st.file_uploader() function in app.py was running into the label value error (see this issue).