[s2s] : Ratify Verification flow

[GarethCOliver]

This issue is tracking consensus on the Verification endpoints in the current proposed text.

Currently there are Five proposed endpoints:

• Initiate: Issuer Endpoint to start a verification session and establish a WSK. Optionally provides VerificationData
• Supplement: Issuer Endpoint to provide additional VerificationData
• Notify: Wallet Endpoint used by the Issuer to notify it on Verification Status changing
• Status: Issuer Endpoint to fetch the current Verification Status
• Cancel: Issuer Endpoint to terminate a Verification Session

On completion transition to the 'credential/provision' endpoint.