Syntax could be better but works in a standards-compliant way.
```
codec := webrtc.NewRTPH264Codec(webrtc.DefaultPayloadTypeH264, 90000)
codec.RTPCodecCapability.RTCPFeedback = []webrtc.RTCPFeedback{{
Type: "nack",
}, {
Type: "nack",
Parameter: "pli",
}}
```