Fixes Ref column values imported from not resolving correctly (#2446)

## Context

When Grist imports an Airtable reference column, it can pass the
Airtable record ids to the sandbox to get looked up in the target table.

If the Airtable reference column only allows a single reference, we
import it as a Ref column instead of a RefList. However, when the
"lookup records by id" value was sent to the sandbox for a Ref column,
it was still an array. The sandbox wouldn't perform a lookup, and just
leave the Airtable record is in the cell as alt text.

## This commit

This passes the column type to the reference resolver, and allows the
reference resolver to decide to send either a single record id to lookup
or an array of them.