Fix "too many open files" error when running integration tests (apache#800)

This was due to the fact that each token fetch created a new client, and the client wasn't closed.

Author: adutra

integration-tests/src/main/java/org/apache/polaris/service/it/env/PolarisClient.java

@@ -115,7 +115,7 @@ public String obtainToken(PrincipalWithCredentials principal) {
 
   /** Requests an access token from the Polaris server for the given {@link ClientCredentials}. */
   public String obtainToken(ClientCredentials credentials) {
-    return polarisServerManager().accessManager().obtainAccessToken(endpoints, credentials);
+    return polarisServerManager().accessManager(client).obtainAccessToken(endpoints, credentials);
   }
 
   private boolean ownedName(String name) {

integration-tests/src/main/java/org/apache/polaris/service/it/ext/PolarisServerManager.java

@@ -51,8 +51,8 @@ public interface PolarisServerManager {
    */
   Server serverForContext(ExtensionContext context);
 
-  default PolarisAccessManager accessManager() {
-    return new IcebergTokenAccessManager(createClient());
+  default PolarisAccessManager accessManager(Client client) {
+    return new IcebergTokenAccessManager(client);
   }
 
   /** Create a new HTTP client for accessing the server targeted by tests. */