rename `credential_configurations` to `credential_configuration_ids`

[Sakurann]

I was explaining the concept of the naming of the parameters and the concept of three different layers to the engineering team (in the spirit of issue #91 and recent parameter name changes) and it was pointed out that there is the discrepancy in the naming of the credential_configurations parameter in the Credential offer and credential_configuration_id in the authorization details, even though they point to the same identifier.

I think it makes sense to align the Credential Offer parameter name to credential_configuration_ids. For the sake of preventing breaking changes, if there is enough agreement this should probably go into the spec before ID-1.