Add P-chain dynamic fees execution

[StephenButtolph]

Why this should be merged

This adds the dynamic fee logic into P-chain block building and execution.

How this works

1. When the time is advanced, the capacity is increased and the excess is decreased.
2. After the time has been advanced, the fee calculator is created to charge the minimum fee based on the excess.
3. The complexity of every transaction in the block is accumulated.
4. The capacity is decreased and the excess is increased based on the accumulated complexity of the block.
   a. This will return an error (and cause block verification to fail) if the capacity would go negative.

How this was tested

• Added unit tests for new functionality.

[StephenButtolph]

This file needed to be moved because of the introduction of the "github.com/ava-labs/avalanchego/vms/platformvm". As that caused a circular dependency.