[FEATURE] [API] Add a new REST API endpoint for schedule

[theimpulson]

Is your feature request related to a problem? Please describe.

The current volunteer API doesn't provide a way to get the current/past schedule.

Describe the solution you'd like

This can be something like /schedule/{year} which returns the JSON schedule for COSCUP that year. If the schedule doesn't exist, a 404 error or empty JSON can be sent as a response. It will be nice if this endpoint doesn't require any authorization.

Describe alternatives you've considered

Downloading the session JSON manually and inflating it in the app database. This however will require an update to the app every time a new schedule is released/changed.

Additional context

Having the session list will allow us to show the volunteers' ongoing sessions and make the app a bit more usable without any kind of login.