Allow CustomPropSocket as a group interface type

CustomPropSocket never opts into is_valid_interface_type, so
interface.new_socket rejects it and a node group can't expose a
Custom Property input/output. The group flatten already handles the
link fine (verified on 4.5.5 and 5.2.0 -- the property lands on the
target bone with default/min/max/soft intact when routed through a
same-colored StringSocket stand-in), so this just adds the same
opt-in flag every other interface-legal socket declares.

socket_definitions.py

@@ -1308,6 +1308,7 @@ class CustomPropSocket(MantisSocket):
     bl_idname = 'CustomPropSocket'
     bl_label = "Custom Property"
     color_simple = cString
+    is_valid_interface_type=True
     color : bpy.props.FloatVectorProperty(default=cString, size=4)
     input : bpy.props.BoolProperty(default =False,)