Get token from AuthenticateAsync

**What version of Docker.DotNet?:**

fork from main as of 2/14/2021 (commit 8e3cf4f7)

**Problem:**
`ISystemOperations.AuthenticateAsync` does not provide a way for the caller to get the token that was returned in the REST response.

Its signature is `Task AuthenticateAsync(AuthConfig authConfig, CancellationToken cancellationToken = default(CancellationToken))`, but it should probably be `Task AuthenticateAsync(AuthConfig authConfig, CancellationToken cancellationToken = default(CancellationToken))` instead.

**Other details:**
I'm happy to submit a PR, but what's this project's preferred way to handle changes that would break binary compatibility? Just changing the return type would be breaking. An overload with an `out` parameter isn't possible because it's an async method. Are you OK with adding an `Authenticate2Async` method or similar?