Plugin-ize authentication backend

[soapy1]

Context

Currently, conda-store supports a few different auth backends. However, projects that use conda-store may require extending the available authentication options. For example, nebari defines it's own KeyCloakAuthentication class. It would be better if nebari could install an authentication plugin for keycloak instead of defining the class within the config file.

Value and/or benefit

conda-store consumers can extend conda-store to use custom authentication providers.