feat: Extended auth handler configuration

[mattkram]

Summary

Currently, the conda auth handler implementation inside anaconda-auth assumes a set of rules when performing token retrieval, given a request URL. In general, we assume the channel and auth domains match (which is sufficient for most self-hosted scenarios), or leverage a set of hard-coded values for production domains. We also support two types of credential for legacy support. However, in certain scenarios, it may be necessary to decouple the request URL from the auth domain lookup URL, or to explicitly specify which type of credential to use.

Here, we implement an extended token-lookup function, leveraging optional fields that can be passed in conda's channel_settings. In doing so, user's can explicitly override default behaviors to match the channel with the token lookup behavior.

We do this by adding the optional auth_domain and credential_type fields, for example:

channel_settings:
- auth: anaconda-auth 
  channel: https://some-special-server.anaconda.com/*
  auth_domain: anaconda.com
  credential_type: api-key

In this case, an API key stored under the domain anaconda.com would be retrieved. Valid credential types are "api-key" and "repo-token".

Considerations

This solution relies on the fact that conda's channel_settings is a plain dictionary object. There is precedence for additional fields (such as username in the conda-auth plugin ref). But there is also an opportunity for plugins to declare their own "extra" config schema.

Retrieval of channel settings from within the auth handler is currently challenging and replicates similar functionality as conda itself. I have opened a feature request issue in conda, which can help to simplify our implementation down the line if merged.

Additional note

This PR is designed to stack on top of #196. During implementation, I found it necessary to require refactoring the token lookup logic, which ultimately makes this PR much simpler.