handle watermark in backfill

[yuhao-su]

          A random thought, if a watermark column exists as the first column in the pk, can we emit the watermark when backfill? Even maybe we can do some optimization for backfill with that watermark?

Originally posted by @st1page in #7790 (comment)

[chenzl25]

I found that it is almost impossible to emit a watermark even when the first column of the backfill executor is a watermark column because the value of the watermark column keeps increasing, so the backfill existing data will usually have a value lower than the upstream watermark. That is to say, no watermark should be sent downstream while performing backfill.