proProjector: Add ES checkpoint storage mode

[bartelink]

The initial implementation of the projector wires checkpoint storage for CosmosDB

This PR adds wireup to instead retain checkpoints in EventStoreDB

As covered in the comments, there are negatives to doing this:

• each checkpoint write will trigger an event to be projected (feedback), so a projector will continually be processing
• Equinox does not currently implement adjustment of stream metadata to retain only the most recent event