Expose the Azure storage table's "Timestamp" for the models where Timestamp should be user-accessible and remove the Timestamp field from models that did not sign up for it.
The behavior where Timestamp is only set by Azure Storage is kept.
Add support for sharding across multiple storage accounts for blob containers used for corpus management.
Things to note:
1. Additional storage accounts must be in the same resource group, support the "blob" endpoint, and have the tag `storage_type` with the value `corpus`. A utility is provided (`src/utils/add-corpus-storage-accounts`), which adds storage accounts.
2. If any secondary storage accounts exist, they are used by default for containers.
3. Storage account names are cached in memory the Azure Function instance forever. Upon adding new storage accounts, the app needs to be restarted to pick up the new accounts.
We need to move to supporting data sharding.
One of the steps towards that is stop passing around `account_id`, rather we need to specify the type of storage we need.
