Add reactive database support for `Shopping Cart` Service

[sokimaaa]

Context

• For time being we are going to use postgres for this service, but replace with NoSQL database in future
• We need to set up library for persistent logic
• We need to provide kick-off configurations, e.g. database url, password and so on
• We need to code the entity model regarding ER diagrams
• Be align with Reactive Approach
• Consider using the same username/password for all services, e.g. username=postgres and password=postgres

Acceptance Criteria

• The persistent logic library is added
• The project runs with connection to database
• The Entity Model is created and fully align with ER diagrams

Technical Details

• Explore what is R2DBC: https://medium.com/@dasi.rajesh08/spring-boot-crud-operations-using-reactive-postgresql-driver-r2dbc-c7bf282b73b7

[andrey-kiyas]

Link to pull request for the current task