Fix/lambda data saving #86

[HappyDevs1]

This pull request includes the following:

• Refactoring the AWS lambda function to aggregate sales data by month and saving data to S3 bucket as json file instead of RDS to save on cost
• Adding logic to read data from S3 bucket using the express server and returning the response as json format
• Removing any logic related to saving sales data into AWS RDS

Closes #86