Modify the `erigon seg step-rebase` tool to operate on erigondb.toml

[wmitsuda]

We currently have some tooling to manipulate the stepsize of an existing datadir:

erigon seg step-rebase --new-step-size=N --datadir <DATADIR>

but it assumes the current stepsize/frozensteps settings are the defaults (it usually is, but should be noted as something that is error prone) and outputs some overriding flags that should be added to the erigon start script (missing that would lead to unpredictable results).

With #18082 those manual steps should not be necessary anymore, and erigon seg step-rebase should operate on erigondb.toml, i.e., read/write settings on it.