GUACAMOLE-524: Add LDAP attributes to credentials.

AuthenticationProviderService gets LDAP attributes
from confService and queries the LDAP server to
find values on user for specified attributes.
Added a Map<String, String> to Credentials named ldapAttrs
and a getLDAPAttributes() and setLDAPAttributes() to
manipulate ldapAttrs on credentials. Once
AuthenticationProviderService gets the values for the
LDAP attributes it sets ldapAttrs on the credentials object.

guacamole-ext/src/main/java/org/apache/guacamole/net/auth/Credentials.java

@@ -22,7 +22,7 @@ package org.apache.guacamole.net.auth;
 import java.io.Serializable;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpSession;
-
+import java.util.Map;
 
 /**
  * Simple arbitrary set of credentials, including a username/password pair,
@@ -72,6 +72,29 @@ public class Credentials implements Serializable {
      */
     private transient HttpSession session;
 
+    /**
+     * Arbitrary LDAP attributes specified in guacamole.properties
+     */
+     private Map<String, String> ldapAttrs;
+
+     /**
+      * Returns the lDAP attributes associated with this set of credentials.
+      * @return The LDAP attributes Map associated with this set of credentials,
+      *         or  null if no LDAP Attributes have been set.
+      */
+     public Map<String, String> getLDAPAttributes() {
+         return ldapAttrs;
+     }
+
+     /**
+      * Sets the LDAP attributes associated with this set of credentials.
+      * @param attributes The LDAP attributes to associate with this set of
+      *                   credentials.
+      */
+     public void setLDAPAttributes(Map<String, String> attributes) {
+         this.ldapAttrs = attributes;
+     }
+
     /**
      * Returns the password associated with this set of credentials.
      * @return The password associated with this username/password pair, or