Support configuration via the command line

[jwodder]

Add command-line options for setting the following:

• The DANDI instance to provide a view to
• The API token to use to access the instance
  • Should this be read from an environment variable instead? Should there be an option to control whether to fetch the token from the environment vs. use no token?
• The S3 bucket on which Zarr entries are stored
  • Alternatively, try to derive this from Zarr assets' S3 contentUrls
• The port to listen on
• The IP address to listen on?
• The URL path under which to serve the WebDAV view?