[14.0][IMP] account_invoice_import: add support for discount

[alexis-via]

Add support for discount on lines in the multi-line create scenario

[alexis-via]

I adapted account_invoice_import_invoice2data tests because, surprisingly, account_invoice_import_invoice2data was already sending the 'discount' field in the pivot dict.

[alexis-via]

/ocabot merge minor

[OCA-git-bot]

What a great day to merge this nice PR. Let's do it! Prepared branch 14.0-ocabot-merge-pr-1017-by-alexis-via-bump-minor, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at fea6c93634ed4726a76fcbd7f3d619b88b896b0e. Thanks a lot for contributing to OCA. ❤️