Issue 85 add availability api endpoints

[EdenWuyt]

Change Summary

/rooms/{id}/availability: Returns availabile slots for a single room grouped by date
/rooms/availability: Returns paginated results of availability for multiple rooms (in the sense that whether they can be booked within a time range) => which I think it is useful after starting to implement #24

Change Form

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• The pull request title has an issue number
• The change works by "Smoke testing" or quick testing
• The change has tests
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Other Information

Assumption:

• Start datetime and end datetime of a room must be on the same date. (I guess a meeting room will not open from or until midnight)
• Slots returned will only include time later than now.
  To do:
• handling EXDATE / RDATE based on Add logic in booking to validate if the room is available #83

Related issue

• Resolve Add availability API endpoints #85

[EdenWuyt]

I will add another endpoint /rooms/availability to quickly get all rooms' availibity with a date time filter.