Table - fix typing for onSort function

[adhityamamallan]

Summary

• Make onSort required only if table config has at least 1 sortable column
• Fix Table to render unsortable cells if sortBy is not passed when sortable is true for a column
• Add helper type TableConfig for readability
• Add unit tests for TableSortableHeadCell
• Mock TableSortableHeadCell out in Table test

Test plan

Type checks succeeding locally + tested by modifying the config