[MERGE] base: custom models improvements

Custom model improvements needed for a Studio task:
- allow ordering custom models by something else than their id
- allow a basic `group_expand` functionality for custom models
  allowing 'stage-like' behaviours (e.g. a `read_group` on `x_stage_id`
will return all the stages, including empty ones)
- small fixes detected during development:
  - SQL identifiers escaping
  - correct reset of the registry after `SingleTransactionCase` and `SavepointCase` tests
  - correct removal of a custom class from the registry

Part of Task 2091654
Related to odoo/enterprise#7326

closes odoo/odoo#43981

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>

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