Optional auth crate

[fredr]

I'm wondering if you would be open to making the auth crate optional, so that is possible to provide your own auth implementation?

We use our own auth crate for all of our other gcp-related auth. It implements a sans-io approach so that we can control what http client is being used so that we can have the same http client throughout all of our projects.

We are trying to keep our dependency tree down as much as possible (as you might have noticed from my previous contributions), and for us it would be great to be able to use the same gcp auth implementation for all of our dependencies as well.

If you are open to it, I'm of course happy to do the work and implement it. But I wanted to check first that you are willing to merge such a feature.

I haven't looked into how to implement it, but I'm thinking that I would put the auth crate/implementation behind a feature (that is part of the default features), and then use some a trait or something that can be implement by other auth crates.

But I'll properly look into how to implement it if you think this is something you would accept.

[yoshidan]

I am open to your idea. I look forward to your Pull Request.

[fredr]

Closing as this was fixed in #105, thank you :heart: