Make gas price service optional for node operators

[xgreenx]

In FuelLabs/fuel-vm#888 we add the max percent on which gas price can change from block to block on consensus level. It allows all nodes in the network have the same rule regarding estimation of the worst gas price.

The regular nodes in the network don't need to run an algorithm to determine the next gas price; they can use the percent to calculate it based on the previous gas price.

We need to make gas price service and all related fields optional for the node operator to use. Mostly, only the block producer should use them.