Decouple Code Suggestions engine classes to better follow the clean architecture approach

Problem to solve

Our existing ModelEngineCompletions and ModelEngineGenerations classes implement the full workflow that includes pre-processing, model call, and post-processing. This approach makes it difficult to test individual components and is not flexible if we need to switch off pre-processing, for instance - example.

Existing architecture:

Proposal

The initial plan was to follow the clean architecture approach - https://blog.cleancoder.com/uncle-bob/2012/08/13/the-clean-architecture.html. Please, consider changing the existing architecture to:

The new architecture allows us to build as many use-cases (CodeCompletions, CodeGenerations) as we need with different configurations - with pre-/post-processing, with post-processing only, etc. We also avoid connecting our components into the chain which complicates testing.