[IMP] l10n_it*: Split Payment

When an Italian company bills a PA business (for example they're selling cleaning services for a public building) the PA business will send the VAT to the Tax Agency themselves, while generally the VAT is collected and sent to the Tax Agency by the buyer (the PA business). This is done to avoid VAT fraud as the PA doesn't trust the business will actually pay the VAT. That's very common in Italy.
A new module will be created (`l10n_it_edi_pa`) in the next commit to add fields that are required to handle Split Payment, and it will be merged in master.

- New Split Payment related accounts are created: 2607, 2608
- New tax report data to target the VE tax chart grid
- Split Payment account.taxes are Groups of Taxes whose children target VE38 tax grid
- account_tax's l10n_it_vat_due_date is no more, we base ourselves on the VE38 tax grid
- The Group of Taxes includes normal VAT and a reversed VAT entry (for sale)
- Tax checks on the invoice now also check group of taxes with flatten_taxes_hierarchy()
- New Split Payment tax group has been added to build the correct totals in the move form view
- account.taxes and account.fiscal.position has been added to change from VAT to VAT Split Payment automatically when you select a res.partner that features the fiscal position.
- The Fiscal Position also has the law-required note that has to be featured on invoices that use Split Payment
- A PA business demo partner is added to showcase the new fiscal position
- `l10n_it_stock_ddt` tests are minimally modified because the Form component didn't let you use 'like' in the move form view

Task link: https://www.odoo.com/web#id=2823645&model=project.task
task-2823645

Part-of: odoo/odoo#132026