Consider setting ReservationInfo for static reservations

Currently, we do not set ReservationInfo for static reservations. Moreover, we suggest that frameworks determine if a resource offer contains dynamically reserved resources by checking whether the ReservationInfo field is set. This means that if we want to add new reservation-specific information (e.g., labels), we can't put those fields into ReservationInfo if we want them to be supported by static reservations.

Proposed plan:

• Continue to add reservation-specific information to ReservationInfo, which means it won't be supported for static reservations
• Set ReservationInfo for both static and dynamic reservations
• Introduce a new enum to describe the type of the reservation, and encourage frameworks to use this field to check for whether a resource offer has a reservation.

The tricky part is the backward compatibility story.