Introduce CheckEndpointConnector, implement for OpenShift connector.

[dhaus67]

Overview

Add the ability to check the endpoints used by the connectors. The goal is to give clients the possibility to check whether a connector's endpoints are reachable.

What this PR does / why we need it

The ultimate goal behind is to avoid potential misconfiguration when creating a new connector. Right now, it wouldn't be possible to detect this at the configuration point, but rather only when using the connector to login if you i.e. use dex within your application.

Ideally, we'd like to check this during creation.
Since this would be a major, undesirable change to introduce this during the Open method, I've opted to add a new interface which connectors may implement, if they want to offer the functionality to provide such a method.

Alternative to the interface choice, we could also decide to make this method exclusive to the openshift connector, similar to the OpenWithHTTPClient, to only expose it for library usage.

Special notes for your reviewer

Does this PR introduce a user-facing change?

Added CheckEndpointConnector, provide the ability to test

[dhaus67]

This might be interesting to you @nabokihms , as it is somewhat similar to #2430 .

[nabokihms]

Thank you for the PR. It makes sense to me. Generic OIDC connector also works the same way. I will take a look through the code and give detailed feedback next week.

[nabokihms]

I've looked through this PR, and, in my opinion, it is not right to add more public interfaces only to use Dex as a library. The best for me is to add optional endpoints health check to the Open method. It just makes more sense.

@sagikazarmark, could you also please have a look at this code?

[dhaus67]

@nabokihms thanks for the feedback.
I can see why we don't want to expose a public interface for that as well. I've gone the way you mentioned with regards to the optional endpoints health check via options that can be passed to OpenWithOptions, allowing for customization within library usage.

Let me know what you think about it.

[nabokihms]

Thank you for the quick fix, @dhaus67. I'm still waiting for the second opinion here. We need to figure out what to do with this (and probably with further) Openshift Connector improvements.

[dhaus67]

@nabokihms @sagikazarmark any update on your end regarding this?