IssuerService: implement Credential vertical

[paullatzelsperger]

Feature Request

implement in-mem and SQL store variant for credentials as well as a service layer that creates the transactional boundaries.

The service should:

• query credentials (by participant ID and via query)
• revoke credential
• suspend + resume credential (possibly empty for now)
• get credential status
• create credential: when a credential is issued to a participant, the credential metadata is recorded and linked to the participant. This method is not accessible via API.

Note that for Credential revocation we'll need implementations of a status list (either BitStringStatusList or StatusList2021). Those will come later, for now we simply maintain the credential status in the database.

Note also that the IssuerService will not track the actual (signed) credential, but only its metadata.

Which Areas Would Be Affected?

spi, issuerservice-core, sql-store