Generic job for backfilling desired sharding keys

What does this MR do and why?

Add a generic background migration for backfilling desired sharding keys

As part of defining sharding keys for every cell local table, many tables will require their sharding key to be backfilled via a foreign table.

Inheriting from a generic job means the query doesn't need to be rewritten for each column that is backfilled.

The query and specs are adapted from !136800 (merged), which is a previous example we did manually (without using GitLab Housekeeper).

!144128 (closed) is an example of a housekeeper MR that will queue a job that inherits from this one.

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