Make channels.update use the update options pattern

[tudddorrr]

Channel update API consolidation

• Collapsed two separate request classes (ChannelsUpdateRequest, ChannelsUpdateOwnerRequest) and a branching update method into a single UpdateChannelOptions parameter with nullable fields, simplifying the call site and eliminating internal conditional logic

Partial update support

• Replaced UnityEngine.JsonUtility with a new JsonUtils.BuildObject helper that omits null fields from serialized JSON, enabling selective partial updates (only changed fields are sent) rather than always sending every field

API ergonomics improvement

• Changed Channel.Update() from accepting positional parameters to a single options object, making callers more readable and less brittle when adding/removing update fields