How do I use AVAssetWriter to encode 4:4:4 HEVC video?

I'm using AVAssetWriter to encode a HEVC video, but I don't want chroma subsampling. Is this supported?

This video will not need to play in real-time; it will be used only for transcoding.

iOS has a way of specifying the profile and level, AVVideoProfileLevelKey, but the only option in the documentation for HEVC is auto level.