sso-auth: add support for individual e-mail address authentication

[starcraft66]

Is your feature request related to a problem? Please describe.
Currently, it is only possible to whitelist an entire domain of addresses, which is convenient for organizations, however it is not so convenient for single users wishing to authenticate oAuth on a public domain such as gmail.com.

Describe the solution you'd like
It would be nice if there was a configuration option to specify a list of individual e-mail addresses to whitelist, or a path to a file containing a list of emails to whitelist, in addition to optionally also specifying whitelisted domain(s).

Describe alternatives you've considered
I haven't considered any alternatives whitelisting methods because Google is the only oAuth provider currently supported.

Additional context
This feature was already available in oauth2_proxy via the "authenticated-emails-file" command-line flag.

[tahoward]

I would like to see this implemented as well. Use case would be for personal services that don't require $5 a month for G Suite.

[starcraft66]

I did not create a pull request myself as I am not fluent in Go, however I don't see this feature being a priority task for the buzzfeed developers as it is of pretty much no use to them. In the mean time, I have found another similar project that offers this feature as of a few days ago: https://github.com/thomseddon/traefik-forward-auth

[shrayolacrayon]

Hi! Thank you for opening. This is something that we do not internally have the bandwidth to implement but we are always open to merging in open source contributions! To me this would look something like creating a new provider that uses htpasswd in its implementation.

[tahoward]

I have a PR in the works. Going to try deploying tonight for some integration testing.

[mreiferson]

See #113

[benjsto]

This was done in #113.