ingest: allow consuming from an S3 ledger backend

[mollykarcher]

What problem does your feature solve?

We'd like a way for people to consume data from an S3 datastore that was exported via Galexie. Support for that feature is in-work here: https://github.com/stellar/go/pull/5708

What would you like to see?

Allow the BufferedStorageBackend in the ingest library to be configured to read from S3, instead of just GCS.

[urvisavla]

The S3 backend will work with BufferedStorageBackend as long as it implements the Datastore interface, can be configured using datastore_config{} and is added to the factory here without any changes: https://github.com/stellar/go/blob/master/support/datastore/datastore.go