Add support for collective transfer (Sammelüberweisung)

[salimhb]

Todo

• add new API-Endpoint (e.g. /credit_transfers) that accepts an array of credit params
• add db table transaction_groups to save transaction grouping
• add model TransactionGroup to handle execution,
• move code from https://github.com/railslove/ebicsbox/blob/main/box/models/transaction.rb to new Model ☝ to keep Transaction slim
• extend https://github.com/railslove/ebicsbox/blob/main/box/business_processes/credit.rb to accept multiple credits as an array and create TransactionGroup
• enhance https://github.com/railslove/ebicsbox/blob/main/box/jobs/credit.rb to handle TransactionGroup and create Transactions and execute the batch
• add specs for new code
• migrate data and drop excessive columns from transactions table (🛑 NOT REVERTABLE, let's discuss)
• do it for direct debits as well

Good news
✅ it seems like no changes in https://github.com/railslove/epics or https://github.com/salesking/sepa_king are needed