do we need `mandatory` in VCI metadata?

[Sakurann]

          Is there any use case for using a `mandatory` set to `false` (or absent)? I.e. is including a _claims description object_ for a given claim path with `mandatory` set to `false` (or absent) the same as not including that _claims description object_?

Originally posted by @pmhsfelix in #276 (comment)

[danielfett]

At least we need to be precise about what it means when the claims description object is not present.

[patatoid]

I think mandatory attributes can be useful since it enforces the integrity of the verifiable credential in some cases, some vc do not have any sense with null values for certain claims.

[Sakurann]

labelling 1.1 because, if we decide to, we can remove this post 1.0 in a backward compatible manner