use rapidjson to write ann files in coco streaming export

[Eldies]

Summary

• Switched to rapidjson in COCO streaming exporter for better performance
• Removed unused json-stream dependency

Measurements and comparisons, ~1.8x speedup with the same memory use.

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2025 CVAT.ai Corporation
#
# SPDX-License-Identifier: MIT

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud