feat: add dynamo block index migrator

[alanshaw]

Adds a lambda that for a given CAR and CAR CID, will:

1. Read multihashes for the CAR either by reading it's CARv2 side index or (if missing) read the CAR itself
2. Batch get from dynamo existing items for each blockmultihash+carpath
   1. Change carpath for each item to be auto/carpark-prod-0/<CAR_CID>/<CAR_CID>.car
   2. Batch write new items with the changed carpath
   3. Batch delete the old items with the old carpath