Bug 27636: Add payin_amount to Koha::Account

Bug 27636: Add payin_amount to Koha::Account

This patch adds a `payin_amount` method to the Koha::Account as a modern
replacement to the `pay` method.

The new method uses the `add_credit` and `Koha::Account::Line->apply`
methods internally for consistency and returns the created credit
Koha::Account::Line result.

Test plan
1/ Run t/db_dependent/Koha/Account.t

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>