issue 106 secure secrets storing

[gonczor]

• Basic get_secret function
• Define BaseSecretsRetriever abstract class
• Implement AWS SSM secrets retriever
• Implement environment variables secrets retriever
• Create Retriever factory
• Retrieve secrets using dedicated retriever class

closes #106

[gonczor]

Overall: it might be a good idea to store all of the vars in SSM and pull them all the time? This way I won't have to update 2 separate task definitions in ECS.