pulse-server: block arbitrary filter-graphs

Add a special 'blocked' spa-libs value that returns EPERM when trying to
load the factory.

Only allow loading the LADSPA filter.graph nodes for the LADSPA sink and
source. The most problematic part is the pipe filter, that allows it to
spawn arbirary programs as part of the filter.graph.

You can add a filter-graph to any stream with stream_props.

doc/dox/config/pipewire.conf.5.md

@@ -358,6 +358,8 @@ factory-name and the plugin where the factory can be found.
 Factory names can contain a wildcard to group several related factories into one
 plugin. The plugin is loaded from the first matching factory-name.
 
+A special `blocked` value for the plugin disables the factory-name.
+
 ## Example
 
 ```
@@ -374,6 +376,7 @@ context.spa-libs = {
     api.jack.*      = jack/libspa-jack
     support.*       = support/libspa-support
     video.convert.* = videoconvert/libspa-videoconvert
+    #filter.graph   = blocked
 }
 ```