inbound: support S3 compatible storage layers

[adamdecaf]

PayGate Version: v0.8.0

What were you trying to do? A few people in the community have asked if we can support downloading/storing inbound files (which consists of incoming transfers, corrections, returns, prenotes, etc) in locations other than the local filesystem. This is useful to populate an S3-type bucket for other processing routines.

What did you expect to see? Setting a config like the following:

odfi:
  storage:
    # "local" is the current block
    remote:
      connectionURI: <string> # s3:// gcs:// or similar URL

The Go Cloud SDK has blobstore support which we can leverage for the major cloud providers. I'll need to double check if this supports other S3-like storage engines (like minio)