Add reference fields to Google Sheets plugin

[madebyisaacr]

Add support for using reference and multi-reference fields in the Google Sheets plugin.

Items would be referenced by their slug, or a comma-separated list of slugs for multi-references.

[jonathanstanley]

fyi https://github.com/anny-co/framer-plugins/blob/39065a51e95b88c3fbb2053da505df1a00617c8f/plugins/google-sheets-import/src/csv.ts#L176-L206