Allow list_files and set_file in user's container

[dfguerrerom]

I'm developing a Solara version for one of our sepal_ui applications, and the final piece is to work with the user-files sepal module—specifically, being able to list and set files.
Right now, I'm using the download endpoint to retrieve files, but I'm still missing endpoints for listing and setting them.

@lpaolini I've started a draft PR (#341) to get the conversation going.