Implement a comptroller

[PaulRBerg]

As discussed in sablier-labs/flow#1, we are preparing to expand the scope of the Sablier V2 protocol. We are considering implementing different "species" of streams in the future, a decision that challenges some of the assumptions we've had when we started working on this repository (v2-core).

Even if there will be different species of streams, it is likely that some configuration values will be common across all contracts (a great insight that @razgraf had during our latest in-person meeting). The initial values that are common today are the (i) protocol fees and (ii) the flash loan configuration, but there may be more in the future.

Therefore, we should implement a "comptroller" contract in charge of the protocol configuration, which would be connected to all Sablier V2 contracts by a storage value that can be edited by the contract owner. By making the value editable, we make it possible to extend the comptroller in a backward compatible manner.