Make activity resolution payload methods return a reference

[lucperkins]

What was changed

Changes the interface provided in #814 to return a Payload reference, as suggested in this Slack message.

Why?

Provides slightly more informative error messages and makes the methods no longer require ownership of ActivityResolution, which is generally an ergonomic win in Rust.

Checklist

1. Closes no issues (it's a small change so I opted not to create an issue first)

2. How was this tested: the standard battery of CI tests

3. Any docs updates needed? I don't believe so

[lucperkins]

On second though, I don't love this because it makes an expression like this no longer compile:

let payload = ctx.activity(activity_options).await.success_payload_or_error();

You can make this a two-liner:

let res = ctx.activity(activity).await;
let payload = res.unwrap_ok_payload();

But that feels like an ergonomic downgrade to me, so I'm going to close this in favor of the status quo.