Keep track of total tuple count at creation

What does this MR do?

When we created batched migrations, we would like to keep track of the expected amount of total tuples the migration is going to touch. Typically, this is the total relation size we're working on.

In this change, we introduce a column batched_background_migrations.total_tuple_count. This is being populated from pg_class.reltuples for the given table, if this information is available. We also "backfill" this information for already in-flight migrations on .com.

Relates to #326994 (closed)

Does this MR meet the acceptance criteria?

Conformity

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