tonic provides no way to disable server-only dependencies

[djc]

If you pull in tonic 0.7 as a dependency, the transport feature (which is enabled by default) pulls in the axum dependency. It appears only servers need the axum dependency, so for downstream users who only need client code, this is unnecessary.

[djc]

I finally tried using #960 and I don't think it's an adequate fix. First of all, the tls feature still pulls in transport, so by default if you pull in tls or one of the features that depend on it, you'll still get the server-side dependencies. That part can actually be worked around by depending on the other required features directly, but then I noticed that the entire tonic::transport module is unavailable if the transport feature is disabled, so I don't have access to things like Error, Channel or Endpoint from that module.

Should I submit a PR replacing the use of transport internally with channel for client-side or both-side items?