Allow arbitrary keys on form components

[prestoncabe]

In certain situations, we need to be able to set a key that isn't derived from an eligibility check:

• anticipated pieces of data for checks that don't exist yet (perhaps not required, but speaks to Sara's comment)
• frontend-only questions; these are questions that help the flow of the screener (e.g. used to hide/show another question) or are used in aggregate to fulfill a form input (e.g. by using the Expression component)

From original slack message: https://codeforphilly.slack.com/archives/C0AJUER0J3Z/p1773714651598909?thread_ts=1773681931.890359&channel=C0AJUER0J3Z&message_ts=1773714651.598909