Add new fields to a sheet

[pkpp1233]

It'd be great to add new fields to a sheet using the API. This would, ideally, be done in one of two ways:

1. Write mode for metadata API. Specifically, an endpoint to update the fields in a sheet.
2. When creating a record, send a parameter (eg create_fields=true), that if the new record has fields not in the sheet, those fields will be created.

This enables templates or importing data without having to manually set up the sheet structure. Eg - imagine an integration that lets someone import their CRM into a sheet. Currently, they'd need to manually create each column, then do the import.