Implement Channel API w/Type Builder

[nathanielford]

Motivation

The Channel object needs a stable API with which users can easily create and manipulate a channel. The current object works, but requires something more ergonomic from both a user and maintenance perspective. This draft is to demonstrate how a type-builder value accumulation API might look like.

This is just a draft, meant to elaborate on one approach.

Solution

This uses a type builder to allows a user to do the following:

let creds = get_channel_credentials();
let custom_rt = get_custom_runtime(); // Instance of `GrpcRuntime`

let channel = Channel::builder("http://[::1]:50051")
    .with_credentials(creds)
    .with_runtime(custom_rt)
    .build();

Similarly, if the tokio feature flag was implemented and the user wanted to use the default runtime they could do the following:

let creds = get_channel_credentials();
let channel = Channel::builder("http://[::1]:50051")
    .with_credentials(creds)
    .build();

Additional options could be chained on to the builder, as normal.