Add ability to provide raw SQL as returning and update to #upsert_all

[palkan]

Revamp of #36636.

Summary

Original discussion: #35077 (comment)

• Added ability to use custom SQL in returning: option:

Article.insert_all(
 [
    { title: "Article 1", slug: "article-1", published: false },
    { title: "Article 2", slug: "article-2", published: false }
  ],
  # Some PostgreSQL magic here to detect which rows have been actually inserted
  returning: Arel.sql("id, (xmax = '0') as inserted, name as new_name")
)

• Added new update_sql: option to specify SQL fragment to use when updating rows on conflict:

Book.upsert_all(
  [{ id: 1, status: 1 }, { id: 2, status: 1 }],
  on_duplicate: Arel.sql("status = GREATEST(books.status, EXCLUDED.status)")
)

Another example by
@boblail:

ExceptionReport.upsert(new_report, on_duplicatel: Arel.sql("count = count + 1"))

/cc @rafaelfranca

[rafaelfranca]

Instead of update_sql does it makes sense to use the on_duplicated option? If it is a string we use it as the SQL to update on conflict.

[rafaelfranca]

And to avoid SQL injection we just only accept Arel.sql both here an in the returning option.