Rename Rust kit attribute macro to worker

[Angelmmiguel]

In the initial versions of this project, we used handler as the attribute macro for the Rust kit. However, our documentation references worker as the right term for this kind of Request / Respose functions.

This will be a breaking change. Since we are still defining the final API, it's fine to apply those at this moment.

To complete this task you will need to:

• Rename the kits/rust/handler project to kits/rust/worker. This includes all references in Cargo.toml, kits/rust/Cargo.toml, kits/rust/handler/Cargo.toml and any place in which we import it.
• Change the macro attribute name and all its references in kits/rust/worker
• Update the exported name in kits/rust/src/lib.rs
• Update all the examples in the documentation
• Update the examples in the examples folder