[16.0][IMP] account_statement_import_online_gocardless: use a hashing strategy to produce the unique_transaction_id

[JordiBForgeFlow]

When using account_statement_import_online_gocardless with Wise, on a given payment Wise creates two transactions. One for the payment and one for the fee.

Each payment using Wise generates 2 transactions:

1. Payment transaction: contains a unique transactionID
2. Fee transaction: contains a unique transactionID, and adds to the field entryReference the transactionID associated with the payment.

So we cannot rely on the field 'entryReference' to construct the 'unique_import_id'.

In general one realizes that it can be very hard to decide which field will contain the unique transaction identifier. So, why not use all possible fields and produce a hash of them all?

This is what this PR does.

[JordiBForgeFlow]

@pedrobaeza @etobella What do you think about this?