When calling the serve flag currently, it tries to support everything all at once:

• serving EPUB files from a directory
• allowing arbitrary EPUB files to be streamed over HTTP/HTTPS
• streaming from S3
• and streaming from GCS

This feels a bit much and shouldn't be the default state when you launch the HTTP server through this command.

Instead, we should:

• default to a directory
• but allow an optional flag to set which URI schemes are supported

For this flag, we could use the following list of values: file, http, s3 and gcs