Add on_video_send callback to esp_webrtc

[recursivecodes]

Description

Modified version of PR 83. Addressed concerns about proper place to put on_video_send callback.

This will enable SEI insertion in the video frames by providing a callback to hook into each frame.

Related

PR #83

Testing

Tested locally with SEI insertion.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.