Add personal_namespace_id column to events

What does this MR do and why?

This new column will be used for a composite sharding key on group_id/project_id/personal_namespace_id.

This MR also prepares async index creation for index on the new column, as it takes between 40 and 70 minutes to be created in the database testing pipeline. The index will be created synchronously in a subsequent MR.

See #462801 (closed).

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Related to #462801 (closed)