feat: Make async_trait optional

[AaronDewes]

Motivation

Using async fn in traits has been stabilized in recent Rust version. Because of this, the dependency on async_trait is no longer necessary with recent Rust versions. Removing unneeded dependencies can reduce compile times and help reduce the risk of supply chain attacks.

Solution

This implements a feature async_trait for tonic and tonic_build. To avoid breaking any existing things, this feature is enabled by default.

[matze]

Doesn't this require #1556 in case that feature is disabled?

[AaronDewes]

Yes, you're right, I also noticed that while trying, that's why it's a draft. I'll have a look at integrating these changes soon.