feat: add optional reused frame callbacks for zero-copy observers

[wnnce]

Summary

This PR adds opt-in reused observer callbacks for RTC raw video, encoded video, and selected
audio frame callbacks.

The goal is to reduce per-frame allocations in high-frequency observer paths. The existing
observer callbacks keep their current copy-based behavior. The new callbacks expose SDK-owned
buffers through unsafe.Slice and let callers decide when and how to copy into their own
memory.

What Changed

New observer callbacks

• VideoFrameObserver.OnReusedFrame
• VideoEncodedFrameObserver.OnReusedEncodedVideoFrame
• AudioFrameObserver.OnReusedRecordAudioFrame
• AudioFrameObserver.OnReusedPlaybackAudioFrame
• AudioFrameObserver.OnReusedMixedAudioFrame
• AudioFrameObserver.OnReusedEarMonitoringAudioFrame

Callback behavior

• Existing callbacks are unchanged and still return copied Go-owned buffers.
• New reused callbacks expose temporary zero-copy views of SDK-owned buffers.
• Reused callbacks are prioritized when both reused and legacy callbacks are set.
• OnPlaybackAudioFrameBeforeMixing is intentionally unchanged.

Examples

Added dedicated examples for the new APIs:

• go_sdk/examples/send_recv_yuv_reused
• go_sdk/examples/recv_h264_reused
• go_sdk/examples/recv_pcm_reused

These examples demonstrate the intended usage pattern:
copy inside the callback if the data must outlive the callback.

API Contract

The reused callbacks expose SDK-owned memory and therefore have stricter lifetime rules:

• the frame buffer is only valid during the callback
• callers must copy the data inside the callback before storing it or passing it to another
  goroutine
• buffers must be treated as read-only

This keeps the zero-copy optimization isolated to explicit opt-in APIs without changing the
safety guarantees of the existing callbacks.

Compatibility

This change is backward compatible.

• Existing callback names and behavior are preserved.
• Existing applications do not need to change anything.
• Only applications that explicitly adopt OnReused... callbacks opt into the new memory
  contract.