Add support for dynamic ldap filter expressions

"Dynamic" is a more of a overstatement when describing this commit but
the current implementation is just the start. Once our ldap protocol stuff
supports our filter implementation this will be vastly improved.

refs #8365

library/Icinga/Protocol/Ldap/Expression.php

@@ -0,0 +1,30 @@
+<?php
+/* Icinga Web 2 | (c) 2013-2015 Icinga Development Team | GPLv2+ */
+
+namespace Icinga\Protocol\Ldap;
+
+class Expression
+{
+    protected $value;
+
+    public function __construct($value)
+    {
+        $this->value = $value;
+    }
+
+    public function setValue($value)
+    {
+        $this->value = $value;
+        return $this;
+    }
+
+    public function getValue()
+    {
+        return $this->value;
+    }
+
+    public function __toString()
+    {
+        return (string) $this->getValue();
+    }
+}

library/Icinga/Protocol/Ldap/Query.php

@@ -306,6 +306,19 @@ class Query
         return $paginator;
     }
 
+    /**
+     * Add a filter expression to this query
+     *
+     * @param   Expression  $expression
+     *
+     * @return  Query
+     */
+    public function addFilter(Expression $expression)
+    {
+        $this->filters[] = $expression;
+        return $this;
+    }
+
     /**
      * Returns the LDAP filter that will be applied
      *
@@ -318,12 +331,16 @@ class Query
             throw new Exception('Object class is mandatory');
         }
         foreach ($this->filters as $key => $value) {
+            if ($value instanceof Expression) {
+                $parts[] = (string) $value;
+            } else {
                 $parts[] = sprintf(
                     '%s=%s',
                     LdapUtils::quoteForSearch($key),
                     LdapUtils::quoteForSearch($value, true)
                 );
             }
+        }
         if (count($parts) > 1) {
             return '(&(' . implode(')(', $parts) . '))';
         } else {