Pipe improvements, misc cleanup

* Pipe improvements; better filter code, pipeline etc.
* Moved commands in PDO plugin to dedicated namespace

runtime/functions.php

@@ -2,6 +2,8 @@
 
 use Spark\Commands\Command;
 use Spark\Environment\Environment;
+use Spark\Pipe\Filters\FilterInterface;
+use Spark\Pipe\Filters\PhpFilter;
 use Spark\Resource\ResourceType;
 use Spark\SparkApplication;
 
@@ -87,7 +89,7 @@ function read_config($file=null) {
 
 $FILTERS = [];
 
-function register_filter(string $name, callable $filter) {
+function register_filter(string $name, string|callable $filter) {
     global $FILTERS;
     $FILTERS[$name] = $filter;
 }
@@ -98,8 +100,18 @@ function get_registered_filters(): array
     return array_keys($FILTERS);
 }
 
-function get_filter(string $name): ?callable
+function get_filter(string $name, array $args=[]): null|FilterInterface|callable
 {
     global $FILTERS;
-    return $FILTERS[$name]??null;
+    $filter = $FILTERS[$name]??null;
+    if (is_string($filter) && class_exists($filter)) {
+        $filter = new $filter;
+    }
+    if (is_callable($filter)) {
+        $filter = new PhpFilter($filter);
+    }
+    if ($filter instanceof FilterInterface) {
+        $filter->setArguments($args);
+    }
+    return $filter;
 }