Add deployment script for stBTC and MezoAllocator contract upgrades (#…

…698)

These are upgrade scripts for stBTC and MezoAllocator contracts that
will be executed on Sepolia and Mainnet.

The solution is using proxy upgrade helpers function that will deploy
implementation contract and prepare transaction that will be executed by
the governance to upgrade the implementation on the proxy contract.

It requires changes from
keep-network/hardhat-helpers#54 which branch is
set in package.json as version of the package. We need to update the
version of `keep-network/hardhat-helpers` dependency once it is
released.