Support naming Tokio tasks

[adamchalmers]

Tokio has a task::Builder interface which lets you give tasks names. It's useful for identifying tasks in the Tokio console.

This API is only available on "tokio_unstable" Rust flag, and tokio crate feature tracing. So Dropshot would need to check for the "tokio_unstable" in RUSTFLAGS and conditionally enable the tracing tokio feature.

This builder API is open to change (obviously, given the "tokio_unstable" name). So maybe y'all aren't open to supporting this feature right now. But it would be a nice feature for people using tokio console.

Engineers at KittyCAD are open to doing the implementation of this, but just opening this issue to discuss if y'all are open to the idea :)