Update gRPC multixplex example for axum 0.7 and hyper 1

[FlyinPancake]

• I have looked for existing issues (including closed) about this

Feature Request

Update example rest-grpc-multiplex

Motivation

Currently, the example code in https://github.com/tokio-rs/axum/tree/main/examples/rest-grpc-multiplex is commented out
and marked as TODO.

The code in the example is not really straight forward for beginners and can be a good starting point for learning exercises.

Proposal

Update the code to be compatible with axum 0.7 and hyper 1.

[takkuumi]

I'll try to make it work this week.

[FlyinPancake]

Has there been any progress on this?

[jplatte]

There has been progress on hyperium/tonic#1670, and once that is merged and released, this sounds be pretty simple to do.