[IMP] migration: allow to add external migration steps

[pro-odoo]

With this commit, it's now possible for an external integration to add custom migration steps. Before this commit, one could only add migration by creating its own migration system.

It will be useful for pivot migration, as some migrations are defined in odoo and some migrations will have to be defined in spreadsheet. But as spreadsheet migrations are run before odoo migrations, it could generate some conflicts.

Task: 4052912

Description:

description of this task, what is implemented and why it is implemented that way.

Task: : TASK_ID

review checklist

• [ ] feature is organized in plugin, or UI components
• [ ] support of duplicate sheet (deep copy)
• [ ] in model/core: ranges are Range object, and can be adapted (adaptRanges)
• [ ] in model/UI: ranges are strings (to show the user)
• [ ] undo-able commands (uses this.history.update)
• [ ] multiuser-able commands (has inverse commands and transformations where needed)
• [ ] new/updated/removed commands are documented
• [ ] exportable in excel
• [ ] translations (_t("qmsdf %s", abc))
• [ ] unit tested
• [ ] clean commented code
• [ ] track breaking changes
• [ ] doc is rebuild (npm run doc)
• [ ] status is correct in Odoo

[robodoo]