Add event publishing for IssuanceProcess state transitions

[admetaq]

At the moment, the IssuanceProcessManagerImpl does not publish any events when an IssuanceProcess changes state.

This feels inconsistent with the behaviour of other state machine managers in the codebase, such as the TransferProcessManagerImpl, where state transitions trigger events that adopters can subscribe to.

One practical use case is observability and auditing. For example, a credential issuer may want to track when a credential was successfully delivered, rejected, or failed during issuance. As far as I know, the only current way to react to those changes is by polling the IssuanceProcessStore.

It could be useful to introduce event publishing for IssuanceProcess transitions, following the same pattern already used in the other state machine managers.

[paullatzelsperger]

this is a good point. converting this into an issue.