Use Keycloak's HTTP client for proxy granting ticket callback

 src/main/java/org/keycloak/protocol/cas/endpoints/AbstractValidateEndpoint.java

@@ -27,15 +27,25 @@
import java.util.UUID;
import java.util.regex.Pattern;
import java.util.stream.Collectors;
import org.apache.http.client.HttpClient;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.utils.URIBuilder;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.HttpResponse;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.util.EntityUtils;
import org.keycloak.connections.httpclient.HttpClientProvider;

public abstract class AbstractValidateEndpoint {
    protected final Logger logger = Logger.getLogger(getClass());
    private static final Pattern DOT = Pattern.compile("\\.");
    private static final Pattern SEPERATION_PATTERN = Pattern.compile("--");
    private static final int PGT_CALLBACK_TIMEOUT_MS = 5000;
    private static final RequestConfig PGT_CALLBACK_REQUEST_CONFIG = RequestConfig.custom()
            .setConnectTimeout(PGT_CALLBACK_TIMEOUT_MS)
            .setConnectionRequestTimeout(PGT_CALLBACK_TIMEOUT_MS)
            .setSocketTimeout(PGT_CALLBACK_TIMEOUT_MS)
            .setRedirectsEnabled(false)
            .build();
    protected KeycloakSession session;
    protected RealmModel realm;
    protected EventBuilder event;
@@ -227,16 +237,23 @@
        String pgtId  = getPGT(session, clientSession, pgtUrl);

        try {
            HttpResponse response = HttpClientBuilder.create().build().execute(
                new HttpGet(new URIBuilder(pgtUrl).setParameter("pgtIou",pgtIou).setParameter("pgtId",pgtId).build())
            );
            HttpGet request = new HttpGet(new URIBuilder(pgtUrl).setParameter("pgtIou",pgtIou).setParameter("pgtId",pgtId).build());
            request.setConfig(PGT_CALLBACK_REQUEST_CONFIG);

            if (response.getStatusLine().getStatusCode() != 200) {
                throw new Exception();
            HttpClient httpClient = session.getProvider(HttpClientProvider.class).getHttpClient();
            HttpResponse response = httpClient.execute(request);
            try {
                if (response.getStatusLine().getStatusCode() != 200) {
                    throw new Exception();
                }
            } finally {
                EntityUtils.consumeQuietly(response.getEntity());
            }

            this.pgtIou = pgtIou;
        } catch (Exception e) {
            // The PGT was not delivered, so do not leave it usable
            session.singleUseObjects().remove(pgtId.substring(0, pgtId.indexOf("--")));
            event.error(Errors.INVALID_REQUEST);
            throw new CASValidationException(CASErrorCode.PROXY_CALLBACK_ERROR, "Proxy callback returned an error", Response.Status.BAD_REQUEST);
        }