[FIX] ir_model_fields: register selection options

Before this commit:
        -> Debug mode
        -> Settings
        -> Database structure
        -> Fields
        -> Any selection field
=> The field `selection` of the ir.model.fields form view does not
display the selection options of the field being viewed, this is because
the selection field is not registered at `_reflect_field_params` of
`ir.model.fields`.

After this commit:

The field is properly registered; for Selection fields with static
options, these are shown as-is, for fields with a lambda function as
options, the string 'function' is displayed, and for fields using a
function name as a string, the same string will be displayed.

Fixes #28360

closes odoo/odoo#31207

odoo/addons/base/ir/ir_model.py

@@ -742,6 +742,10 @@ class IrModelFields(models.Model):
     def _reflect_field_params(self, field):
         """ Return the values to write to the database for the given field. """
         model = self.env['ir.model']._get(field.model_name)
+        selection = getattr(field, 'selection', None)
+        if not isinstance(selection, list):
+            # only reflect the selection of a selection field if it's a list
+            selection = None
         return {
             'model_id': model.id,
             'model': field.model_name,
@@ -759,6 +763,7 @@ class IrModelFields(models.Model):
             'readonly': bool(field.readonly),
             'required': bool(field.required),
             'selectable': bool(field.search or field.store),
+            'selection': selection,
             'size': getattr(field, 'size', None),
             'translate': bool(field.translate),
             'relation_field': field.inverse_name if field.type == 'one2many' else None,