Genesis chainconfig cleanup

[ceyonur]

Why this should be merged

1- This adds a new upgrade config networkUpgradeOverrides to override any avalanche network upgrade. With this upgrade config and skip-upgrade-check flag set to true, one can modify the timestamp of activated upgrade. This will come handy if a subnet has missed the upgrade (failed to update nodes) and as a result their chain has become unsyncable with newer versions. In this case networkUpgradeOverrides can be used to set a later time for upgrades. I.e:
upgrade.json:

{
  "networkUpgradeOverrides": {
    "durangoTimestamp": 1710415083
  }
}

2- Default Network Upgrades are introduced. nil and blank values in network upgrades (eth forks + avalanche upgrades) will now default to values. Meaning that it won't be possible to deactivate an existing upgrade via nil values.

3- This PR also removes OptionalNetworkUpgrades since now we can modify all NetworkUpgrades.

How this was tested

Unit tests + local tests

How is this documented

it will be documented in docs repo.