Create external_data_handler module

[YgLK]

Create external_data_handler module which will be used to collect historical data from external sources.

• Create scripts to download data from external sources
• Validate if downloaded data has appropriate format and is free of errors
• Refactor downloaded data and insert it into the database
• Create .sh script which will initiate cron job downloading the data once per set time interval

After app deployment, script downloading external data will be running in the background from the app container.