Objects returned by getMessageIterator() now include source and destination IDs

Each migration using a SQL-based id_map plugin has two database tables: a map table and a message table. Corresponding rows in the two tables have the same source_ids_hash value. Each row in the map table should have a corresponding row in the map table, but not not every row in the map table has to have a corresponding row in the message table.

The getMessageIterator() method from Drupal\migrate\Plugin\migrate\id_map\Sql now returns the source and destination IDs, from the corresponding row of the map table, along with the source_ids_hash and other values from the message table.

Before example of message object:

$message = (object) [
  'msgid' => '1',
  Sql::SOURCE_IDS_HASH => 'abcdefg123456789',
  'level' => '1',
  'message' => "'a message' is not an array",
]

After example of message object:

$message = (object) [
  'src_name' => 'source_message',
  'dest_config_name' => NULL,
  'msgid' => '1',
  Sql::SOURCE_IDS_HASH => 'abcdefg123456789',
  'level' => '1',
  'message' => "'a message' is not an array",
]