Payment reconciliation identifiers at Order level

[nickevansuk]

additionalProperty should be available on the Order or Payment level as well as OrderItem, for scenarios where payment reconciliation metadata is required at this level. Note that schema.org already allows for this, so it is permissible within the current spec, though not explicit.

[nickevansuk]

Suggest additionalProperty on Payment is used, as this is already the case in the models - replacing the OrderItem level property.

[nathansalter]

Agreed, very useful to have a loose framework when discussing payment metadata