UI: Add initial UI support for ID pointers custom properties.

Customprops to IDs are supported since years through code, but were
never exposed directly in the UI of customporperties.

This commit mainly:
* Adds a new `DATA_BLOCK` type to UI customprops types.
* Exposes the existing `id_type` settings to python API.

Pull Request: https://projects.blender.org/blender/blender/pulls/110458

scripts/modules/rna_prop_ui.py

@@ -80,6 +80,7 @@ def rna_idprop_ui_create(
         description=None,
         overridable=False,
         subtype=None,
+        id_type='OBJECT',
 ):
     """Create and initialize a custom property with limits, defaults and other settings."""
 
@@ -91,11 +92,16 @@ def rna_idprop_ui_create(
     proptype, _ = rna_idprop_value_item_type(default)
 
     if (proptype is bool) or (proptype is str):
-        ui_data = item.id_properties_ui(prop)
         ui_data.update(
             description=description,
             default=default,
         )
+    elif proptype is type(None) or issubclass(proptype, bpy.types.ID):
+        ui_data.update(
+            description=description,
+            default=default,
+            id_type=id_type,
+        )
     else:
         if soft_min is None:
             soft_min = min
@@ -156,6 +162,7 @@ def draw(layout, context, context_member, property_type, *, use_edit=True):
 
         to_dict = getattr(value, "to_dict", None)
         to_list = getattr(value, "to_list", None)
+        is_datablock = value is None or isinstance(value, bpy.types.ID)
 
         if to_dict:
             value = to_dict()
@@ -178,6 +185,8 @@ def draw(layout, context, context_member, property_type, *, use_edit=True):
             props = value_column.operator("wm.properties_edit_value", text="Edit Value")
             props.data_path = context_member
             props.property_name = key
+        elif is_datablock:
+            value_column.template_ID(rna_item, '["%s"]' % escape_identifier(key), text="")
         else:
             value_column.prop(rna_item, '["%s"]' % escape_identifier(key), text="")