Get configuration file

1. GET https://opensolr.com/solr_manager/api/get_file
2. Parameters:
   1. ​email - your opensolr registration email address
   2. api_key - your opensolr api_key
   3. index_name - the name of the index
   4. file_name - the name of the config file you wish to get, without the file extension
   5. file_extension - the extension of the config file you wish to get, without the period

3. Example: https://opensolr.com/solr_manager/api/get_file?email=PLEASE_LOG_IN&
   api_key=PLEASE_LOG_IN&core_name=my_solr_core&file_name=schema&file_extension=xml