[IMP] base_import: support multiple xlsx sheets

Excel can store data in multiple tables, each is called a sheet. Odoo
was only importing the first sheet making the process to import a file
containing multiple sheets cumbersome.

It is now possible to select the sheet in the import options. By default
the first sheet is selected.

closes odoo/odoo#40728

Task: 2043768
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>