public MultiSiteOAuth2ClientOptions addAllowedIssuer(String allowedIssuer) {
            this.allowedIssuers.add(allowedIssuer);
            return this;
        }
Simplified factory to create an {@link io.vertx.ext.auth.oauth2.OAuth2Auth} for OpenID Connect.
<p>
Important modification by msavy: to allow internal/external distinction without freaking out, we have
{@code config.getAllowedIssuers().contains(issuerEndpoint)} with user-provided acceptable issuers.

@author <a href="mailto:plopes@redhat.com">Paulo Lopes</a>
gateway/platforms/vertx3/vertx3/src/main/java/io/apiman/gateway/platforms/vertx3/api/auth/KeycloakDiscovery.java