capella: add withdrawal boost factor constant

[q9f]

A WITHDRAWAL_BOOST_FACTOR should always be 1 but can be used to inflate total supply through withdrawals in tests or testnet environments.

Opening this up for discussion. Context eth-clients/goerli#129

cc @parithosh

[ralexstokes]

@lightclient raised this griefing vector:

I deposit 32 goETH and withdraw 320 goETH; rinse and repeat until I control an arbitrary amount of stake and then do what I want to the network, e.g. keep BASE_FEE unacceptably expensive

this is a nice idea to leverage the CL as a "minting" layer but I don't see a way around this particular issue

even w/ some kind of "lock up" period, it would just shift the timing of the attack, not its existence

[dapplion]

@ralexstokes before considering this PR it's necessary to model the exponential supply growth of Goerli for a reasonable timeframe

[pk910]

I think it's even more problematic as anyone can deposit more than 32 ETH to their validator.
So a abusive user could just deposit his whole balance to a validator to get it back multiplied by the factor.

That issue could be mitigated by an additional WITHDRAWAL_BOOST_LIMIT, that limits the amount of funds where the factor applies:

if amount <= WITHDRAWAL_BOOST_LIMIT:
    amount *= WITHDRAWAL_BOOST_FACTOR

But it makes it more complex and doesn't fix the problem that @ralexstokes mentioned.

I'm also not sure if this doesn't break reward calculations for staking pools. They're probably calculating things based on the real validator balance and don't take care of such factors.
Therefore this factor introduces a "custom path" for reward calculations - that's something we wanted to avoid in testnets?

[q9f]

closing as per discussion during today's CL call. focus should be now to prepare application teams for sepolia and holesovice.