fix(rtp): improve timestamp accuracy for video

[andygrundman]

Description

Instead of using now() when the RTP packet is created, use the earlier packet->frame_timestamp that we're already collecting for host latency stats. This timestamp is more accurate to when we captured the frame, and the same timestamp value is shared by all RTP packets that make up the same video frame. Duplicate frames without capture timestamps use the ratecontrol timestamp.

Type of Change

• [X] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Dependency update (updates to dependencies)
• [ ] Documentation update (changes to documentation)
• [ ] Repository update (changes to repository files, e.g. .github/...)

Checklist

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have added or updated the in code docstring/documentation-blocks for new or existing methods/components