No way to get the offered attributes for a credential offer

[TimoGlastra]

Now that the attributes aren't stored anymore in the credential record until after a credential offer has been accepted, there's no easy way to get the credential preview attributes.

E.g. for v1 it looks like this, for v2 it needs a bit different code:

const credentialService = agent.injectionContainer.resolve(V1CredentialService)
const offerMessage = await credentialService.findOfferMessage(route.params.credentialId)

if (!offerMessage) {
 // uh-oh handle error
 }

const attributes = offerMessage.credentialPreview.attributes

I think we should do something like we did in #877 (probably extend the method added in that PR) where we can retrieve the (indy) attributes for a credential exchange. This is only useful for indy credentials, so I'm thinkgin whether we should extend the top level properties:

{
  proposalAttributes: [] 
  proposal: {
     indy: {
       cred_def_id: "cred_def_id",
     }
  },
  offerAttributes: [] 
  offer: {
    indy: {
      cred_def_id: "cred_def_id",
    }
  }
}

Or that we should just add it to the indy data (althoug it won't be strictly the format data then anymore, which it currently is). Both makes sense to me, Only indy uses the attributes, so adding it indy (like we do with the other method (e.g. offerCredential attributes are nested under credentialFormats.indy):

{
  proposal: {
     indy: {
       attributes: [],
       cred_def_id: "cred_def_id",
     }
  },
  offer: {
    indy: {
      cred_def_id: "cred_def_id",
      attributes: [] 
    }
  }
}

I think I'm leaning towards the first approach, but not sure yet.

@jakubkoci, @JamesKEbert, @blu3beri @karimStekelenburg your input is welcome