sso-proxy: remove provider interface

[shrayolacrayon]

What

This is something that was not changed from the original oauth_proxy clone that can be removed. Since we only have one provider for sso-proxy - the SSOProvider, to simplify testing and remove default provider logic, it makes sense to get rid of the interface and have the OAuthProxy struct's provider field be an SSOProvider rather than a Provider interface.

[luisdavim]

won't this block #27 ?

[shrayolacrayon]

This is specifically to remove the provider interface (or consolidate both provider interfaces into one) in sso-proxy. This was something that we have yet to clean up from when we began development from starting off with using oauth2_proxy. sso-proxy's only provider is sso-auth and sso-auth uses a third party authenticator defined by a provider interface. https://github.com/buzzfeed/sso/issues/27 refers to sso-auth's third-party providers.

[luisdavim]

Makes sense, thanks for explaining that.