Tax rate information missing

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### Preconditions

1. Migrate Magento 1.9.4.2 to Magento 2.3.2 (settings and data)

### Steps to reproduce

1. in Magento 1 place order with a taxed item
2. Enable setting Systems > Configuration > Tax > Orders, Invoices > Display Full Tax Summary
3. Migrate settings and data

### Expected result

1. In Magento 1 the specific applied tax rate is shown but is missing from Magento 2

### Actual result

1. see above

### Additional notes
This seems to be intended behaviour?! as the values are simply zeroed out here https://github.com/magento/data-migration-tool/blob/2.3.2/etc/opensource-to-opensource/1.9.4.2/map.xml.dist#L1882

The values are available in the source database. The below seems to work for me using a custom handler:

```

sales_order_tax_item.real_base_amount







```
for each of the 4 fields `real_base_amount`, `real_amount`, `base_amount` and `amount` with base and real being the same (which works in most cases where there is only 1 tax rate applied - see https://github.com/magento/magento2/blob/2.3.2/app/code/Magento/Tax/Model/Plugin/OrderSave.php#L95.

The handler looks like this
```
destination = $destination;
$this->destinationAdapter = $destination->getAdapter();
$this->tableFrom = $tableFrom;
$this->commonIdField = $commonIdField;
$this->fieldCopy = $fieldCopy;
$this->fieldCopyTo = $fieldCopyTo;
}

public function handle(Record $recordToHandle, Record $oppositeRecord)
{
$this->validate($recordToHandle);
$fieldCopyValue = $this->getOppFieldValue($recordToHandle->getValue($this->commonIdField)) ;
$oppositeRecord->setValue($this->fieldCopyTo, $fieldCopyValue);
}

public function getOppFieldValue($id)
{
/** @var \Magento\Framework\DB\Select $select */
$select = $this->destinationAdapter->getSelect();
$select->from(['opp' => $this->destination->addDocumentPrefix($this->tableFrom)], [])
->where('opp.'.$this->commonIdField.'=?', $id);
$select->columns([$this->fieldCopy]);
return $select->getAdapter()->fetchOne($select);
}
}
```

and results in

Start with etc/opensource-to-opensource/1.9.4.2/map.xml.dist around the sales_order_tax_item mappings, then compare the source database fields with Magento 2 output and review Magento/Tax/Model/Plugin/OrderSave.php. Done means migrated orders retain the relevant tax amounts so the applied tax rate information is available as in Magento 1.

Written by the indexing model from the issue text.