[rust] Attribute macros for writing async entry points

Adds a procedural macro crate intended to simplify the boilerplate for writing entry
points that use asynchronous execution, such as `main` or `#[test]` functions

Test: Performed manual testing out of tree.

Change-Id: I8e7341dfc15e2af1d7818a553fee0006bedd7c2e
2 files changed