Refactor ImportPipeline logic in the storage-proxy

## Motivation

### ImportPipeline creation refactoring

- Let’s improve readability by clearly separating the arguments passed to `create_import_pipeline` into those provided at service initialization time and those provided at handler execution time.
- Overall, refactor the code in a way that avoids argument drilling as much as possible.

### Etcs

Let’s refactor the cleanup_stage-related logic by introducing a new class and passing its instance around, and remove that logic from the public methods of `ImportStep`.

JIRA Issue: BA-3884