pulse-server: move extension to modules

Add extension support to modules. This is a list of extension commands
that can be performed on the module.

Remove the custom registry of extensions and make proper modules that
implement the extensions.

This is more in line with what pulseaudio does. The advantage is that the
modules actually show up in the module list and that we can use the
module user_data to implement the extension later.

src/modules/module-protocol-pulse/defs.h

@@ -37,7 +37,6 @@
 #define SCACHE_ENTRY_SIZE_MAX	(1024*1024*16)
 
 #define MODULE_INDEX_MASK	0xfffffffu
-#define MODULE_EXTENSION_FLAG	(1u << 28)
 #define MODULE_FLAG		(1u << 29)
 
 #define DEFAULT_SINK		"@DEFAULT_SINK@"