Make `helix::client::custom` first-class

[Emilgardis]

I want to make the req_*_custom methods more useful, this would provide a way to do no allocation as wanted with #114, provided that the user creates the structs they need.

• Don't place custom behind unsupported
• consider improving ergonomics of how to specify return type. e.g is let resp: MyStruct = client.req_get_custom(req, &token).await?.data(); enough?