[Editorial] Refactor Credential and Deferred Credential endpoint text to be less duplicative

[GarethCOliver]

Currently Credential Request/Responses are defined separately for Credential and Deferred Credential Endpoints, with the Deferred Credential Request/Response re-using a subset of the Credential Endpoint and adding another field.

If we instead defined Credential Request and Response separately to the two endpoints, and simply provided the restrictions (i.e. Credential Request MUST contain proofs. Deferrered Credential Request MUST contain transaction_id), we'd remove duplication, prevent issues where parameters are added to one and not the other and make it easier if we wanted to move to having only Credential Endpoint handle both in the future.

This wouldn't be a technical change, but is a semantic one (i.e. that the request to each endpoint is conceptually the same, and the response from each is conceptually the same). This is most clear on the response, but less so on the request.

I do think it'd make reading and understanding deferred issuance a lot easier so a good simplification to make.

[Sakurann]

started with #583, but eventually resolved by merging #631