Define `indexAllocator` in the specification

[msporny]

The specification currently does not define the indexAllocator property on the /credentials/status. It needs to be defined in the specification.

A PR needs to be raised that says that when the list is created indexAllocator is used, and then must be re-used when status is updated. Status service manages status lists with index allocators.

[ottonomy]

It's in the https://w3c-ccg.github.io/vc-api/#update-status operation, but not clearly defined. It doesn't yet appear anywhere else, such as in

• https://w3c-ccg.github.io/vc-api/#issue-credential (Maybe irrelevant here -- the fully rendered VC is expected to be passed here, it even says including proof, which doesn't seem to make sense for an issuance endpoint, probably just a bug in the schema. Maybe at this point status is already expected to be allocated -- that's how it works in a ~VC-API implementation I was looking at)
• https://w3c-ccg.github.io/vc-api/#create-exchange create exchange (here it could easily be a variable not defined by the spec if a product needs it).

I wonder if this property is implementation-specific and doesn't belong in the spec at all. It seems very specific to certain status methods and products. Other status methods and products definitely won't need this -- after all the statusListIndex is already provided to the Update Status call above.) Some products have an internal understanding of how they assign a particular statusListIndex if that is even a relevant concept for the status method a credential uses. I'd love to see the group consider whether it's possible to treat this as a "variable" that doesn't make it to the specification level but that is used by certain products with certain status methods as desired. If it's included in the spec, it would probably need to be defined much more clearly, including the internal business logic of how a particular indexAllocator relates to the eventual statusListIndex.