fix compile error when tower not updated in Cargo.lock

[tdyas]

Motivation

Fix the following compile error caused by the version of tower in Cargo.lock not having to be at least v0.4.6:

error[E0599]: no method named `layer_fn` found for struct `ServiceBuilder<tower::layer::util::Identity>` in the current scope
  --> tonic/src/transport/service/connection.rs:54:14
   |
54 |             .layer_fn(|s| AddOrigin::new(s, endpoint.uri.clone()))
   |              ^^^^^^^^ method not found in `ServiceBuilder<tower::layer::util::Identity>`

Since the existing version in tonic's Cargo.toml is v0.4.4, cargo did not know to update that dependency automatically if the Tonic developer has an older version in Cargo.lock. Since Tonic is a library, Cargo.lock is not checked in and can vary between developers.

Solution

Specify v0.4.6 for tower to ensure layer_fn is available.

[davidpdrsn]

This was fixed as part of #606