Current-thread executor support

## Feature Request

* Support current-thread executor
* Remove axum dependency

### Crates

* tonic-build
* tonic

### Motivation

I am using tonic in our microservices. But since it assumes multi-threaded runtime, and since we are only using current-thread runtime, Send restriction makes our program very heavy, both for aesthetic, and practical views.

So I suggest to support current-thread executor support.

### Proposal

1. Removal of axum dependency:
* Axum assumes Send. so to achieve current-thread runtime, we should remove axum router.
* Axum is only for routing, and we are using very small part of it. so we don't have any loss if we remove it.

2. Support current-thread version of tonic-build
* We can first start from making the Service trait (?Send). For example, we should use this form of Service trait:
```
#[tonic::async_trait(?Send)]
impl test_server::Test for Svc {
async fn test_request(&self, req: Request) -> Result, Status> {
Ok(Response::new(req.into_inner()))
}
}
```

3. Support the tonic server which can be used with the above generated code
* So we should run that code. No "Send" restriction for the Service trait!