Feature Request: OnlineDDL, support per-migration artifact retention

[shlomi-noach]

Feature Description

This issue was brought up by the community a while back, and today I found its implementation to be a useful feature for testing another issue.

Today we have vttablet --retain_online_ddl_tables=<duration> flag, which controls how long after a migration is complete/failed/cancelled, its artifacts should be kept for safety before being destroyed (ie dropped). The default is 24hour.

The community requested that this variable be configurable per migration. This is doable using ddl-strategy flags. We can e.g. set --ddl-strategy="vitess --retain-artifacts=72h"

Use Case(s)

For when you want to keep migrated tables for a while longer for safety, or vice versa if you want to get rid of claimed space sooner, per migration.