Download Index Backup File

1. ​GET https://opensolr.com/solr_manager/api/download_backup_file
2. Parameters:
   1. ​email - your opensolr registration email address
   2. api_key - your opensolr api_key
   3. ​core_name - the name of the core you wish to download a backup file for
   4. file_name - the name of the backup file, as generated by the get_backup_files_list API method.

3. Example: https://opensolr.com/solr_manager/api/download_backup_file?email=&
   api_key=Please log in&core_name=my_solr_core&file_name=my_backup_file_name