Preserve column header order in data sources

[stevenle]

Describe the bug

When syncing a data source to firestore, the rows are saved as maps. Hash maps in firestore are natively unordered, so the column headers displayed on the data sources page is undeterministic (each refresh may result in a different order).

Proposed Solution: Preserve the column header order somewhere in firestore and have the data source page be able to read the stored sort order.