backupccl: add setting to write files in SQL

This adds a setting -- default off -- to force BACKUP to always ask
KV to return files to SQL to write instead of writing them directly.
This is currently what it does for tenants but not for the system
tenant due to the extra network hop, data copy, cpu and mem overhead.

Release note: none.

pkg/ccl/backupccl/backup_processor.go

@@ -63,6 +63,12 @@ var (
 		time.Minute*5,
 		settings.NonNegativeDuration,
 	)
+	alwaysWriteInProc = settings.RegisterBoolSetting(
+		"bulkio.backup.proxy_file_writes.enabled",
+		"return files to the backup coordination processes to write to "+
+			"external storage instead of writing them directly from the storage layer",
+		false,
+	)
 )
 
 const backupProcessorName = "backupDataProcessor"
@@ -201,7 +207,7 @@ func runBackupProcessor(
 	exportRequestStoreByLocalityKV := storageConfByLocalityKV
 
 	// If this is a tenant backup, we need to write the file from the SQL layer.
-	writeSSTsInProcessor := !flowCtx.Cfg.Codec.ForSystemTenant()
+	writeSSTsInProcessor := !flowCtx.Cfg.Codec.ForSystemTenant() || alwaysWriteInProc.Get(&clusterSettings.SV)
 
 	var defaultStore cloud.ExternalStorage
 	if writeSSTsInProcessor {