DS analyzer: Python support - Manifest scanning: requirements.txt scanning
## Description
This implementation issue covers adding `requirements.txt` manifest scanning capabilities to the Dependency Scanning (DS) analyzer. **This manifest scanning will serve as a fallback mechanism when lock files are not available**
## Scope
**Objective:** Implement manifest scanning functionality for Python `requirements.txt` files
**Key Components:**
* Add `requirements.txt` file detection and parsing logic
* Implement dependency extraction from Python manifest files
* Integrate with existing DS analyzer workflow as a fallback option
* Ensure compatibility with current dependency scanning pipeline
**Acceptance Criteria:**
* DS analyzer successfully detects and processes `requirements.txt` manifest files
* Dependencies are correctly extracted and reported
## Related Issues
* Related MR: https://gitlab.com/gitlab-org/security-products/analyzers/dependency-scanning/-/merge_requests/427/diffs
issue