Use IssuanceServices for signing and verifying data in agent_issuance

[nanderstabel]

Description

At the moment this is how we implement Services related to 'issuer' functionality in several Aggregates command handlers in agent_issuance. The issuer variable can be used to sign and verify data.

               let issuer = {
                    let mut services = SecretManagerServices::new(None);
                    services.init().await.unwrap();
                    Arc::new(services.subject.unwrap())
                };

The downside of this method is that on every call of the command handler SecretManagerServices needs to be initialized again (which means a Stronghold file needs to be loaded which causes significant latency).

Instead, we need to implement a more efficient way of doing this by simply initializing the Stornghold file only once on startup of the UniCore instance, similar to how VerificationServices is implemented in agent_verification.

Motivation

Resolves significant amount of latency.

Resources

n/a

To-do List

• Implement IssuanceServices
• Remove unused code