Support delete/gc the pre-aggregation tables when all related deployments are dropped

**Describe the feature you'd like**

When deployments are dropped, there is no action for the associated pre-aggregation tables.

**Additional context**

We may maintain a counter for every pre-aggregation table. Decrease the counter when one deployment drops. When it reaches zero, users may issue a `gc` command to finally delete the tables.

Look for pre-aggregation table management and deployment lifecycle code. Understand how tables are linked to deployments and where counters could be added. The 'gc' command would need to be implemented to check counters and delete tables when zero.

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