Simplify Api#create_transaction_for by improving the transaction ID generator

[benjaminwil]

In our extensions's Api#create_transaction_for(order) method, we noticed that we need to call OrderTransaction.latest_for(order)&.transaction_id in order to check if we already have a transaction reported.

We think that we can optimize our TransactionIdGenerator.next_transaction_id method to just always call `OrderTransaction.latest_for(order)&.transaction_id so that our API wrapper doesn't need to know about it.

Before:

  latest_transaction_id =
    OrderTransaction.latest_for(order)&.transaction_id

  transaction_id = TransactionIdGenerator.next_transaction_id(
    order: order,
    current_transaction_id: latest_transaction_id
  )

After:

  transaction_id =
    TransactionIdGenerator.next_transaction_id(order: order)