Handle `execute_request_impl(allow_stdin = )`

[romainfrancois]

[adamblake]

@SylvainCorlay is this what would allow inputs via functions like readline()? I have seen this sort of thing used in fairly popular packages (and also in not as popular packages like coursekata) to get user input before:

• doing something that might be destructive
• installing a missing dependency
• performing something computationally intensive

It's not critical for any of our specific use-cases at CourseKata, but I think it is generally going to be expected by other users. If it's hard to accomplish, maybe it would be helpful to have a "phase 1" of the feature where it expresses an error explaining that user inputs don't work. Currently if a package asks for user input it just enters an endless loop of spamming for user input in the kernel logs while appearing busy in the notebook.