Add User Retention API and Streamlit Page

- Introduced a new User Retention API by adding the `user_retention_api` router in the backend application, making it accessible at the `/api/user-retention` endpoint.
- Developed a corresponding Streamlit page for user retention analysis, allowing users to view and filter retention data for hype markets.
- Implemented data fetching from the new API, including features for displaying retention metrics, new trader counts, and lists of retained users.
- Provide options to download data in CSV and JSON formats, along with clear error handling and processing states.

Author: goldhaxx