Guardrails - Balance Submission

As discussed in ETH Balance Submission, in its current form this duty creates a lot of exposure for rETH holders.This section explores removing it by limiting how much rETH price can diverge per update.

Currently the change per update is unlimited. To understand how much the rETH rate could change, lets look at an example: Assume that 1/3 of all validators are slashed around the same time and that also 1/3 of all minipools are slashed. This would mean that rETH loses ~1/3 of it's value over 4096 epochs ( ~18 days). Reducing by 2% every 180 epochs (19.2 hours, the current update frequency) would be enough to capture that change.

Based on this, it appears possible to introduce upper and lower limits for the rate delta. For such a limit to be effective, the contracts would also need to enforce a minimum distance between update. This proof of concept implements those changes. It might make sense to implement the delta as an adjustable setting (maybe guarded by a timelock to ensure this safety feature can't be easily undone).