Support Transaction Policies

[xgreenx]

Overview

Related issue FuelLabs/fuel-core#1404 that should define gas_per_byte.

Related specification change FuelLabs/fuel-specs#514

Add policies to transactions with all limits managed by policies (in preparation for multi-dimensional pricing in the future):

• Move the gas limit to be a policy
• Add witness data limit as a policy

Min fee also should include gas_per_byte * WL(Witness limit). And WL should affect the validity rules of the transaction.
The gas limit should exclude the predicate gas used, and the predicate gas used should be part of the min fee.

[xgreenx]

This change is better to do on top of #533