Add support for form_display_fields in form config field params

[signebedi]

When we have a form lookup, we should enable end users to pass the following field parameter to the form config:

form_display_fields: str | List[str] | None, 

This should be a list of string fields (if the field doesn't exist, then just ignore them) to show as options instead of the (admittedly not very user friendly) document ID.

Originally posted by @signebedi in #150 (comment)

[signebedi]

I think instead of defaulting to None, we should default this to __metadata__document_id, which is the flattened key name for the document ID field. Then, when links_to_form is a string (see #280), we override the options parameter for that field and use these overrides instead (be aware of potential implications this may have for #169. This effectively renders the !all_submissions_<form_name> constructor (see #150) obsolete.