BE: Work Items - visualize records using colors

Summary

This is the backend issue for the requirements found in &10004 (closed). Note that we have a technical spike in #394862 (closed).

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Add new widget to support the color highlight feature present in epics.

• The `color` attribute should be stored in a separate table. Do not add a new column to the issues table
• The corresponding epic feature is behind a feature flag (FF) so we probably want to use a FF for this widget too