Add support for token based authentication

[travishathaway]

What is the idea?

In order to support channel servers such as anaconda.org and prefix.dev, we need to add support for storing and using API tokens.

What should happen?

• Add a new auth handler for token support
• Test usage with anaconda.org and prefix.dev private channels

Additional Context

The way that tokens are submitted vary slight between anaconda.org and prefix.dev.

prefix.dev uses a more standard Authentication: Bearer <token> header, whereas anaconda.org uses a less standard Authentication: token <token> approach.