Implement URL Seeker

[digitaldogsbody]

Add a URL Seeker that uses HTTP Range headers as appropriate to handle the seeking.

Possible approaches:

• Use stream_context_create to inject the headers to a standard fopen() call: https://www.php.net/manual/en/function.stream-context-create.php
• Extended or custom HTTP client that does this anyway
• cURL wrapper

Considerations:

• What to do if the remote server doesn't support Range?
  • If the file is small enough, just retrieve all of it? Let the user configure this via config/constructor?