Skip userinfo if provider doesn't support it.

[jplana]

While integrating the CoreOS Dex oidc (https://github.com/coreos/dex) with teleport we found a problem during the claim validation process:

Dex doesn't implement the UserInfo endpoint, which seems to be optional anyway according to the standard (see dexidp/dex#376).

Teleport auth service crashes when requesting this endpoint as in:

func claimsFromUserInfo(oidcClient *oidc.Client, issuerURL string, accessToken string) (jose.Claims, error) {
        err := isHTTPS(issuerURL)
        if err != nil {
                return nil, trace.Wrap(err)
        }

        oac, err := oidcClient.OAuthClient()
        if err != nil {
                return nil, trace.Wrap(err)
        }
        hc := oac.HttpClient()

        // go get the provider config so we can find out where the UserInfo endpoint is
        pc, err := oidc.FetchProviderConfig(oac.HttpClient(), issuerURL)
        if err != nil {
                return nil, trace.Wrap(err)
        }
        endpoint := pc.UserInfoEndpoint.String()

pc.UserInfoEndpoint is casted to String.

This PR tries to avoid this crash by checking for the UserInfoEndpoint presence in the provider configuration, using just the token claims in this case.

Do you think this would be a good approach?

[russjones]

@jplana Good catch, I'll review your PR shortly.

[russjones]

@jplana Merged, thanks for the fix!

[jplana]

Great! :-)