Scale back use of async-trait where applicable

[aaronmondal]

Since Rust 1.75.0 the language has native support for async fn in Traits for some usecases (https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html).

Dynamic dispatch isn't supported yet (https://blog.rust-lang.org/2023/12/21/async-fn-rpit-in-traits.html#dynamic-dispatch), meaning that a full removal of the async-trait dependency might not be possible at this point.

However, I believe that some uses of aysnc-trait in the NativeLink codebase have become redundant. Removing them is a reduction in complexity and a potential performance improvement.

[adam-singer]

cc: Assigned to @jhpratt