This commit introduces upgradeability of the `RewardsStreamerMP`
contract by leveraging the UUPS pattern.
This means, for deployment, we have to first deploy an instance of
`RewardsStreamerMP` contract as a "template" logic contract and then
create a ERC1967Proxy that points to it.
The proxy ensures the implementation address is stored in a
deterministic storage slot.
This will later be leveraged by the `StakeVault` contract to implement
the functionality to leave the system in case there was a malicious
upgrade.
Closes#22
