Add a mechanism for notifying the wallet about certain events

ODI4VCI would benefit from a mechanism that allows issuers to send notifications to the wallet or its backend to enable:

• The ability to notify the wallet of specific credential issuance process-related events, e.g., to reduce polling requests at the deferred issuance endpoint.
• The ability to inform the wallet or its backend about certain risk-related events, e.g., fraud, key compromise.
• The ability to notify the wallet of specific credential lifecycle-related events, e.g., a credential has been revoked independently from the status mechanism and associated TTL windows, new credential types can be requested, or the underlying credential dataset has changed, which is relevant for many credential types such as age-over-18, marriage, etc.

A potential solution could be based on the OIDF Shared Signals and related IETF standards work as presented at IIW. The slides can be found here.

The solution should support both push and pull notification mechanisms. While the pull mechanism is easier to implement, it has certain limitations. For example, if a wallet must maintain relationships with several issuers, polling event endpoints from those issuers introduces a significant load on the mobile device.

Start by reviewing the notification requirements in this issue and the linked IIW slides on OIDF Shared Signals and related IETF standards. Define a design that covers issuer-to-wallet or backend notifications, supports both push and pull, and addresses issuance, risk, and credential lifecycle events.