feat: automatic revocation index selection

Problem to solve

In the current implementation the index of the credential in the Revocation Status List has to be manually passed and must not be identical to another credential's index in the same list. This index has to be selected with no prior information of issued revocable credentials and this could easily lead to obvious and avoidable failures of the API.

Proposal

We should discuss and implement a strategy for automatic incrementing of the revocation status list index of subsequent issuing of credentials using the same revocation definition. This will also help with the problem of automatically creating a new status list once the limit of the current one is reached.