Add argument ask_special_io to enable skipping user input prompt

[pot-belly-mole]

When requesting one of the input-output tables, like "naio_10_cp1750", in an interactive R session, one currently receives a prompt for user input: "The requested product id is a special input-output matrix. To keep the matrix structure for further use, download it with iotables::iotables_download(). The iotables package is an extension for such cases to the eurostat package. Do you want to stop downloading now? [y/n]"

It was not possible to skip this prompt in an interactive session, which causes issues when used eg. in a pipeline with targets package. Also, there is no (good) way to fake a non-interactive session or otherwise fake input to readline.

This pull request addresses that issue by introducing a new argument ask_special_io to get_eurostat_json, which enables to skip the prompt, if given the value FALSE.

Open questions:

• is the name of the argument ok
• should it be exposed to get_eurostat

I would be fine with this solution that enables skipping the prompt by using the lower level get_eurostat_json function, but of course it could be exposed to the general function as well.