params: Add initial L1 base fee to configuration

[pmikolajczyk41]

1. Bring DefaultInitialL1BaseFee from nitro/arbostypes.
2. Extend genesis config (indirectly with ArbOSInit field) by the InitialL1BaseFee.
3. Add a getter (with a fallback to DefaultInitialL1BaseFee) to ArbOSInit for best-effort backwards compatibility with old genesis.json files.

---

pulled in by OffchainLabs/nitro#4171

[bragaigor]

LGTM

[bragaigor]

LGTM

[diegoximenes]

I arrived at this comment while thinking about how to handle serialized chain config here.

If serialized chain config is provided in ArbOSInit then it will not be possible to have a ArbOSInit.GetSerializedChainConfig that would return a default value, since ArbOSInit doesn't have access to a default chain config.

core.Genesis, that has the ArbOSInit field, has a ChainConfig field that could be used to compute a default serialized chain config.
Nitro allows the core.Genesis.ChainConfig, from genesis JSON file, to be updated BTW, so that is an extra complication.

That said, ArbOSInit will likely not be the best place to retrieve the default InitialL1BaseFee.
This decision is quite coupled with the decision on how serialized chain config will be provided, so ideally they must be done in a single PR.

WDYT?

We can discuss this better in a meeting.