Update code examples related to using `event.credentialRequestOptions.web.VerifiablePresentation`

[dlongley]

          The value of `event.credentialRequestOptions.web.VerifiablePresentation` is a Verifiable Presentation Request (VPR), so we should use the variable name `vpr` instead to help reduce confusion. It is specifically not a `VerifiablePresentation`.

CHAPI extends the Credential Management API by defining a WebCredential that implements the Credential interface (note that Credential is not a VerifiableCredential but a WebIDL interface specific to the browser Credential Management API spec). This is the expected mechanism for extending that spec (other Credential types include FederatedCredential, PasswordCredential, etc.).

Each of these generates C++ (or whatever native code) when implemented directly in the browser, so the number of types of "Credentials" is bounded, unlike how it is unbounded in the Verifiable Credential ecosystem. Also, a WebCredential is not specific to the Verifiable Credentials ecosystem either, allowing for any kind of credential data to be handled by a CredentialHandler. Thus, a WebCredential has its own dataType and data (properties of the WebCredential). The former is always VerifiablePresentation when either getting or storing Verifiable Credentials, as these are transmitted via Verifiable Presentations -- and the Credential Management API's design is such that you "get" and "store" the same "type" of Credential. This means that you either "get" a WebCredential that has, as its data, a VerifiablePresentation, or you "store" one -- at that layer.

Again, in a somewhat awkward fashion, this means that the credentialRequestOptions (again, from the core Credential Management API spec) expresses query information for specific Credential types using the prefix for the extending types as property names: {web: {...}, federated: {...}, password: {...}}. Therefore, when using CHAPI, web is used -- and within web, the specific dataType is further used to continue this design pattern: web: {VerifiablePresentation: {...}}. That does not mean that the value of credentialRequestOptions.web.VerifiablePresentation is a Verifiable Presentation -- as this is not how the Credential Management API data model pattern works. It is, instead, whatever "options" are needed to be passed to find a WebCredential of dataType: VerifiablePresentation. These "options" are defined as a VerifiablePresentationRequest, so the value is a VPR.

I'll convert this into an issue and hopefully the background can be helpful in some way.

_Originally posted by @dlongley in https://github.com/credential-handler/chapi.io/pull/21#discussion_r1297415393_

[brianorwhatever]

yes this naming caused some confusion for me originally so that update would be helpful for new devs. So just to get this straight @dlongley. The web object is describing how to "get" a WebCredential which is by using a VerifiablePresentation. To know how to complete that VerifiablePresentation the VerifiablePresentationRequest is here in this VerifiablePresentation object?

[dlongley]

@brianorwhatever, yes, that sounds right.