Add support for claims specs

[pelle]

We need a way to request various data about a claim. This allows us to specify most importantly an issuer, but also if the claim is required for signup.

A Claims Spec allows you to set specific requirements for the claims you are requesting.

Name	Description	Required
type	The claims type	yes
essential	This claim is essential. A response should not be returned if user does not have this claim	no
iss	Array of DID's of allowed issuers of claims	no
url	URL for obtaining claim	no
reason	Short string explaining why you need this	no

Examples:

{
  type: 'email',
  essential: true,
  iss: ['did:web:uport.claims'],
  url: 'https://uport.claims/email',
  reason: 'Whe need to be able to email you' 
}

[oed]

Do you think something like this will be included in the DID-Auth spec?
cc @awoie

[simonas-notcat]

If the iss field is an array, I think url should also be an array. Or something like this:

{
  type: 'email',
  essential: true,
  iss: [
    {
      did: 'did:web:uport.claims',
      url: 'https://uport.claims/email'
    },
    {
      did: 'did:web:sobol.io',
      url: 'https://sobol.io/verify'
    }
  ],
  reason: 'Whe need to be able to email you' 
}

[awoie]

@oed DID Auth is about exchanging a DID, while the Claims and Credentials people are talking about how to exchange credentials. But I assume that something like this will be included in the spec, yes.

[awoie]

It might be worth looking at the current claim manifest proposal and the way how they structure the request: https://hackmd.io/q-selsVyQQ-1p1LK7mHJxw