siRequirements
These are the requirements for using a SetupIntent.
Only SIRequirements are allowed in this set.
If this is null SetupIntents and PaymentIntents with SFU set are not supported by this LPM. If SetupIntents are supported, but there are no additional requirements this must be an emptySet.
In order to make sure the PM can be used when attached to a customer it must include the requirements of the saved payment method. For instance, Bancontact is not delayed, but when saved it is represented as a SEPA payment method which is delayed. So there must be Delay support in order to meet the requirements of this PM. (There was a consideration of adding a SaveType that in cases where SI or PIw/SFU it would also check the requirements of the SaveType - not sure if the SaveType pi and/or si requirements should be checked).