Prompt for secret values

[anthmatic]

Removes the value argument from hs secrets add and hs secrets update and replaces it with a prompt where the use can input their secret as a masked input like:

? Enter a value for your secret *****

[williamspiro]

Thanks for doing this. cc @TheWebTech this will require some doc updates, both for the CLI docs and sample repos like https://github.com/HubSpot/cms-event-registration#getting-started