Add SkipIssuerCheck option to kubelogin

[hongkunyoo]

Purpose of the feature (why)

I really liked this project. Thanks for your effort.
I would like an option SkipIssuerCheck to skip to issuer check if possible. For example, --skip-issuer-check.
The reason why for this option is, in my K8s environment, I want to expose OIDC provide endpoint only to in-cluster scope for certain users. Which means the OIDC provider endpoint should vary for some users.
For example,

• A user group: https://oidc.mycluster.com (default)
• B user group: http://SVC_NAME.NAMESPACE.svc.cluster.local (for some users)

However, the jwt issuer should be https://oidc.mycluster.com for both case. Since the jwt issuer and OIDC provider endpoint does not match, currently an error occurs. I want to avoid this case.

Would you kindly consider to add --skip-issuer-check option for such case?

Your idea (how)

Add SkipIssuerCheck option in gooidc Verifier.

SkipIssuerCheck: true

kubelogin/pkg/oidc/client/client.go

Line 200 in 42164cb

verifier := c.provider.Verifier(&gooidc.Config{ClientID: c.oauth2Config.ClientID, Now: c.clock.Now, SkipIssuerCheck: true})

I have checked it works well when I add this option. I could just use the downstream and add it only for myself but I'm wondering if it's possible to add an option to the upstream.