Blocking DDL for source backfill

[xxchan]

separated from #15587

• implement track & report feat: track progress for SourceBackfill (blocking DDL) #18112
• improve progress report. See feat: track progress for SourceBackfill (blocking DDL) #18112 (comment) & wg-backfill

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Note: After we support blocking DDL (i.e., make the SourceBackfill's progress tracked), the old non-blocking (inconsistent) style will be lost. In the future, if we use background DDL, a MV doing source backfill will be invisible, same as MV backfill.

We might want to do some enhancements in the future to improve the invisible non-blocking DDL's experience:

• discussion: allow stream query on creating mv?  #12771