Add an Offer::Operation message for Dynamic Reservations

A framework now has a notion of accepting offers that it was given (via acceptOffers) and is able to specify a sequence of operations to perform (via a sequence of Offer::Operation). Launch is one of the possible Offer::Operation and which means LaunchTasks is an alias to a sequence of Offer::Operation consisting of only Launch.

The goal of this ticket is to add Reserve and Unreserve messages as possible Offer::Operation.