[FIX] point_of_sale: group invoice receivable lines by partner

Acronyms:

SRL : receivable lines in the session's account move that balances
the receivable lines in the session's invoices
IRL : receivable lines of the invoices in the pos session

When closing a pos session, a single account move is created to capture
all the transactions, but, invoices are kept (each is an account move).
During the construction of this single account move, SRLs are
reconciled with IRLs.

Prior to this commit, when closing a pos session, SRLs are grouped
by `property_account_receivable_id` of the partners in the invoices.
Because of this, SRLs are blind of the partners that they are linked
to. This causes issues on tracking the 'due' in the partners that are
invoiced using the pos application.

To fix the issue, we now group the SRLs by partner before reconciling
them the the IRLs.

Same concept is employed for receivable lines generated from split
cash payments, we also assigned partner to them if applicable.

closes odoo/odoo#70033

Signed-off-by: agr-odoo <agr-odoo@users.noreply.github.com>