[Generic] Support custom ca_certs bundle in GenericOAuthenticator

[nngo]

Add support to use a custom certificate authority (CA) bundle for talking to GenericOAuthenticator (OAuth2) and overridable via OAUTH2_CA_CERTS environment variable without having to disable certificate validation with OAUTH2_TLS_VERIFY=0 env.

Otherwise get the following error:

• tornado.curl_httpclient.CurlError: HTTP 599: SSL certificate problem: unable to get local issuer certificate

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[nngo]

@consideRatio can you look at this PR?

[nngo]

my end goal is for this updated oauthenticator library to get used by jupyterhub/k8s-hub image, so that can inject my ca-certs there properly while running under K8s and this is the last place to get that to work with using an oauth2 for login for my K8s cert-manager/cluster issued ca-certs.

[jgprogramming]

Is this PR still active? We are considering building a fork just for this feature but we'd rather use a official version.

[manics]

There are some unresolved issues, and this repository has undergone some refactoring so this would need careful testing.

However, we may not need this PR anymore. https://github.com/jupyterhub/oauthenticator/pull/578 adds support for all Tornado request parameters: https://www.tornadoweb.org/en/stable/httpclient.html#request-objects

[consideRatio]

@nngo thank you for working this!!

I'm hopeful that you can use custom certificates without configuring the authenticator specifically, but by configuring tornado running in JupyterHub via #614, and add your certificates to the jupyterhub pod via hub.extraFiles chart config.

It would be great to know if this did the trick for you!