sql/backfill: integrate distributed merge and ingest into legacy index backfill

This is a continuation of issue #158378. That was opened to add distributed merge to the index backfill. We reduced scope of that issue to focus only on integration with the declarative schema changer. This issue is opened to integrate with legacy schema changer.

To get into the path for legacy backfill you need to set this:
```
SET CLUSTER SETTING bulkio.index_backfill.distributed_merge = 'legacy';
```

We already have the first stage integrated, which takes the rows and generates the first intermediate SST.

For test coverage, we can expand existing tests with the `local-dist-merge-backfill-legacy-schema-changer` logic test config.

Jira issue: CRDB-57826

Epic CRDB-62564

Start by reading the existing distributed-merge index backfill integration and the legacy schema changer path. Enable bulkio.index_backfill.distributed_merge with the value 'legacy', then expand the existing tests using the local-dist-merge-backfill-legacy-schema-changer logic test config. Done means distributed merge and ingest are integrated through legacy index backfill with test coverage.

Written by the indexing model from the issue text.