Fix plain-text report for ApiCenterOnboardingPlugin

[waldekmastykarz]

Currently, the generated plain-text report missed an empty line after the non-production APIs section. Also, already registered APIs are not grouped by API.

Current

New APIs that aren't registered in Azure API Center:

https://jsonplaceholder.typicode.com:
  DELETE https://jsonplaceholder.typicode.com/posts/1
APIs that are already registered in Azure API Center:

GET https://jsonplaceholder.typicode.com/posts

Expected

New APIs that aren't registered in Azure API Center:

https://jsonplaceholder.typicode.com:
  DELETE https://jsonplaceholder.typicode.com/posts/1

APIs that are already registered in Azure API Center:

https://jsonplaceholder.typicode.com:
  GET https://jsonplaceholder.typicode.com/posts