Implement an easier way to get Send Serve and Stub

[stevefan1999-personal]

This PR improves upon #475 by observing trait-variant output and closely imitating the behavior without major breaking changes. There is no need to fill in a lot of unnecessary Send Sync bounds after this which clogged up in the original idea for #475 by using a downgrade trick using extension trait. All existing codes including the examples and the tests compiles without any changes.

[tikue]

Would it be possible to use return-type notation to avoid duplicating the trait definition? rust-lang/rust#129629

pub trait SendStub: Stub + Send where <Self as Stub>::call(..): Send {}

impl<S: Stub + Send> SendStub for S where <S as Stub>::call(..): Send {}

Benefits of doing it this way:

• The implementations for Channel and Serve don't need to be duplicated
• the trait method can still be an async fn.

I think the same trick can also be applied to Serve and the proc macro-generated service trait.

Of course, the downside is this is still an experimental feature. But I think I'd rather think about solving this in a forward-thinking way. We can add a feature gate for it here, as well, so that it's only enabled if compiled with a tarpc Cargo feature that enables unstable features.