Convert CH arrays to string in work items table

What does this MR do and why?

Based on performance measurements, filtering array columns is slower than de-normalizing an array into a String (!196808 (comment 2625174627)).

This MR converts the assignee_ids[] and label_ids[] to String columns where the id values are casted to string: /1/2/3/.

What does the MR do:

1. Drop the MV.
2. Drop the columns and re-add them as String.
3. Add the updated MV.

The tables and MVs are not in use yet, so we can safely remove and add the columns.

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