Add a warning dialog in the UI for credential pair keys that are not valid shell variables

[ryan-lane]

A common use of confidant's credentials pairs is to inject them into shell environments. However, credential pair keys aren't limited to strings that are valid for shell environment variable strings. We don't want to limit this because there may be people using confidant in a way that doesn't inject into the shell, and then there's no reason to limit the keys. We do, however, want to want people if they're using keys that may be invalid for their use-case.

So, we should add a warning dialog to the UI that tells people they're using a credential pair key that may limit its usability.