Implement Channel builder

[nathanielford]

Introduces a type-builder Channel builder for creating new Channels, using an idiomatic pattern for adding in configuration options.

Motivation

In order to flexibly and ergonomically provide for channel configuration on construction, the type builder is introduced. This allows users to construct channels in a manner similar to:

let channel = Channel::builder("https://api.darkcave.com:443")
    .credential(my_creds)
    .runtime(my_runtime)
    .override_authority("arrow.wumpus.api")
    // ... any other options.
    .build();  // No error is returned here

Solution

The solution uses a 'type builder' so that users can build up their channel configuration over various calls, and the compiler can catch type issues. This also allows us to encapsulate various ergonomics (such as default selection of runtime). This PR implements the critical values at this time and defers additional optional values not being used until a later point. It also leaves work around the internals of the PersistentChannel/ActiveChannel for a future PR.

Because the public API is being altered, this requires a version bump.