[S2S] - Ratify Identifier Model

[GarethCOliver]

Ratify the currently proposed identifier model. Current proposed text has:

• SessionId as a unique wallet-specified identifier for the Wallet Client Instance's interactions with the Issuer
• CredentialConfigurationId for the issuer's credential issuance configuration
• VerificationId for a particular verification session
• CredentialInstanceId for the identifier of a particular Credential Data set on a particular Wallet Client Instance