Adding springdoc-openapi to the source code

To add OpenAPI-based documentation regarding the external API that's exposed by the product-composite-service microservice, we need to change the source code in two projects:

• product-composite-service: Here, we will set up a configuration of springdoc-openapi in the Java application class, ProductCompositeServiceApplication, and add some general information pertaining to the API.
• api: Here, we will add annotations to the Java interface, ProductCompositeService, describing each RESTful service and its operations. At this stage, we only have one RESTful service with one operation, accepting HTTP GET requests to /product-composite/{productId}, which is used for requesting composite information regarding a specific product.

The actual texts that are used to describe the API operation will be placed in the default property file, application.yml, in the product-composite-service project.Before we can start using springdoc-openapi, we need to...