sql: add SST writer to distributed index backfill merge pipeline

[spilchen]

This change continues the integration of the distributed merge pipeline into the index backfill flow. When enabled, the backfiller writes its KV output batches to SSTs (backed by ExternalStorage) via a new SST sink.

• Processor: Updated rowexec backfiller to route KV batches through the SST sink.
• Job progress: Persist SST manifest metadata in job progress via IndexBackfillMapProgress.

Informs: #158378
Epic: CRDB-48845
Release note: none

[cockroach-teamcity]

This change is 

[fqazi]

Just one issue related to the error handling for progress.

@fqazi reviewed 18 of 18 files at r1, all commit messages.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @DrewKimball, @jeffswenson, @kev-cao, and @mw5h)

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pkg/sql/rowexec/indexbackfiller.go line 425 at r1 (raw file):

			case <-tick.C:
				if err := pushProgress(); err != nil {
					return err

Will this also terminate the ingest thread? I worry that we will just stop tracking progress.

[fqazi]

@spilchen Nice work!

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @DrewKimball, @jeffswenson, @kev-cao, @mw5h, and @spilchen)

[spilchen]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @DrewKimball, @fqazi, @jeffswenson, @kev-cao, and @mw5h)

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pkg/sql/rowexec/indexbackfiller.go line 425 at r1 (raw file):

Previously, fqazi (Faizan Qazi) wrote…

Will this also terminate the ingest thread? I worry that we will just stop tracking progress.

Ah good point. I remember we have this pattern in other places. I'll log a warning and continue if the progress fails . Note: I don't expect this path to fail, that path only exists if we fail to marshal the SST manifests somehow.

[fqazi]

@fqazi reviewed 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball, @jeffswenson, @kev-cao, and @mw5h)