Revisit col_type = "C" now w.r.t `CellData`

[jennybc]

The original class choice for "cell" column type needs to be aligned with what exists now around the googlesheets4_schema_CellData.

Think about full cell data and how it cleaves into read-only vs. writeable fields.

With col_type = "C", there's no way to request "full" data, as opposed to "default", as there is with range_read_cells().