Add a setter for L1 base fee. The trusted oracle can update it over
time. Eventually this will become trustless, but this is a quick and
easy approach for now.

Also add a setter/getter for the per batch overhead and the scalar.

Also emit events for when the values are updated. This will make it much
easier to track historical gas prices over time.

Add tests for new functionality. L2 geth will consume the new value in
the `OVM_GasPriceOracle`

Add getters that make it easy for users to know the L1 costs.
- `getL1Fee` returns the L1 fee given the current L1 base fee known by
  the L2 node
- `getL1GasUsed` counts the bytes and creates a sum for the gas cost of
  submitting the data to L1