Decouple OCI registry requirement on attestations

[migmartri]

Now that we have a new extensions framework and a way to route attestations to one or many OCI registries, we should remove the pre-requisite of using the main OCI registry as storage backend of attestations.

The idea would be to

• Go back to having the attestations (just the envelope) stored in the DB
• Do not send to OCI's main repository

The reasoning for this change is two fold

• There can not be two mechanisms to send attestations to OCI. The extensions framework is the new one and gives more flexibility to our users, so we should leverage it instead.
• The current OCI repository model is starting to look more like a CAS storage backend configuration instead. It's how you configure the CAS and it will evolve over time to having more backends.

### Tasks
- [x] Store and consume the envelope from DB
- [x] Remove code that sends attestations to custom OCI repositories
- [ ] Update documentation