Add the ability to create custom participants to whom transactions can be assigned (even for non-shared budget books)

As of version 1.2.1, users have the ability to manually assign a specific participant of the shared budget book to a transaction. However, this only works if the participant is part of the iCloud share and has accepted it. Nevertheless, some users may want to assign transactions to other participants who are not part of the share, such as their children.

Therefore, a feature should be added to the application that allows users to add additional participants. In addition, the ability to assign transactions to a specific participant can also be enabled for users who have not shared their budget book at all.