Add blob storage service

Adds endpoints for uploading and downloading of blobs. The base of the URL for these endpoints is an existing logistics object of the type cargo:ExternalReference.

Endpoints:

upload: <URL_OF_EXTERNAL_REFERENCE>/upload download: <URL_OF_EXTERNAL_REFERENCE>/download/<FILENAME>

The blobs are stored using the S3 API. Any compatible storage provider may be used.

Related to #116 (closed)