Add new parameter mandate_reference

[stoilkov]

A new optional parameter is available for both Transaction and Subscriptions "mandate_reference" - non empty string.

For transactions:

Update the Transaction model ( add mandateReference String)
Update the create method(s) (as optional parameter)

For Subscriptions

Update the Subscription model ( add mandateReference String)
Update the create method(s) (as optional parameter)
Update the update method (make Subscrition.mandateReference @Updateable )

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