being able to check if a version exist during attestation init

[migmartri]

In some cases we want to make sure that when we start an attestation the version that we are referring to exists already in the system. This is useful, for example, if you are running a packaging task and want to make sure that the backpatch is part of that specific version and don't create a new one.

So this feature will consist of a flag in attestation init like "--existing-version" The backend will do is fail if the version doesn't exist in that case.