Configurable invoicing payment reference

[bjarnef]

Is your feature request related to a problem? Please describe.
At the moment the invoicing payment method by default store a guid (32 characters) as a payment reference (string). However when integrating to different third party systems they might have a character limit on e.g. 20 characters or maybe expect a payment reference as an int.

Describe the solution you'd like
It would be great to configurate the default format of the invoicing payment reference or maybe override the default behaviour of this.