* op-node: Add dedicated PlasmaConfig struct to rollup.Config

This commit adds a dedicated PlasmaConfig section to the rollup
config. This collects all Plasma Mode configuration to the same place
and enables future expansion of Plasma Mode configuration to be confined
to a single location.

In the interim, the op-node can read from both the legacy fields and the
dedicated struct. If both are set, they must be consistent. The genesis
creation tooling currently only writes with the new config.

* Set plasma mode from superchain registry

* Remove LegacyUsePlasma usage