Support scaling of backfill jobs

[kwannoel]

We don't support scaling backfill jobs at the moment. So when we scale backfill jobs running under background ddl, they will trigger an assertion panic as mentioned by #19138.

This is prerequisite of serverless backfill and for improved ux of background_ddl, so we should support it.

[kwannoel]

cc @shanicky, in case you already have ideas about supporting it, or have already started work on it before, so we avoid double work on it.

[xxchan]

This is a little surprising, since I thought it's already supported.
I thought we disallow foreground job to be scaled, but allow background ddls to be scaled.

[kwannoel]

This is a little surprising, since I thought it's already supported. I thought we disallow foreground job to be scaled, but allow background ddls to be scaled.

Unfortunately not. That should be supported, but at that point in time, several logic needs to be changed in the etcd backend and the sql backend, and background_ddl wasn't too high priority. So it's deferred to now.

Priority is increased since the logic will possibly be used for serverless backfill as well.