[FIX] account_facturx: make factur-x xml compliant

The xml we generate for factur-x is no longer compliant with all
the latest standards. With these changes, we are providing the tools
to make it work once again and make sure it is validated by the
factur-x and zugferd validators in all aspect (PDFA/3, XMP, XML)
It should also be valid to be sent to Chorus pro if applicable.

This adds a mapping to the UNECE unit of measure codes for uom, that
are required for factur-x.

It will also calculate a category for each taxes as such:
    - If the tax is an export from EU to outside EU, G.
    - If the supplier and customer are both in EU but different country,
        K
    - If both are in EU, in the same country but the tax amount is 0, E
    - Otherwise, it will be S.
See https://unece.org/fileadmin/DAM/trade/untdid/d16b/tred/tred5305.htm



opw-2714544

closes odoo/odoo#89491

X-original-commit: d25fdafc
Signed-off-by: Laurent Smet <las@odoo.com>