Make RewardStreamer/StakeManager upgradeable via UUPS

[0x-r4bbit]

Once #13 is done, we can look into making this protocol upgradable using standard patterns.
We've had an EIP discussions sessions on upgrade patterns.

One pattern that has been established throughout the years is the UUPS pattern, which allows for having the upgrade logic inside of the implementation contract as well.

It also allows for upgrading to different versions..

This issue is about exploring this pattern in the context of the reward streamer contract, so Status can perform future upgrades on the protocol.

We need to keep in mind though, that StakeVault talks to the reward streamer, and at the time of creating this issue, StakeVault has a fixed API.