Enable dynamic Prefix Handling

[MML-coder]

This is a follow-up (Part-2) to the feature of adding unique prefixes for SyntheticTextItemsGenerator. In Part-1, we decided to add a controllable, different prefix during data generation (see issue #186). The next step is to separate the prefix handling from data generation, so the prefix can be generated, appended, and modified as needed for each backend request. This will help control the prefix cache rate more effectively.