Prompt for variation values when updating feature variables

[emmawillis]

• fix the zod schema so that it knows CreateVariableDTO.defaultValue is optional
• add a prompt for variation values when adding or updating a variable on an existing feature

note: this is a little bit sketchy because when updating the variables list, we need to essentially update the feature variations list as a semi-related side effect. Because the functions on VariableListOptions have to return a valid list option, i couldn't figure out a cleaner way to update the feature variations than to store them on the VariableListOptions class and access them in the feature update command separately. if there's a better way let me know!!

New prompts when editing a variable within a feature update:

this prompt won't show up when updating a single variable outside of a feature, or if you select variables and variations from the chooseFields prompt (since the variations prompt will already ask for the variable values).