Support for making gas price data available to contracts

[miloszm]

Summary

Currently, gas price information is not available to contracts. Contract should be able to call a host method providing such information. Although the host function is also part of PieCrust, Rusk needs to support such functionality in VM.
Contract should be able to get low, average and high values of a gas price.

Possible solution design or implementation

TBD - a way is needed to pass current gas price information to VM and make it available to contracts via a host function.

Additional context

Related to Economic Protocol Scenario 3, issues: #1503, #1603