Enrich deploy method and contract's metadata with optional gas limit and gas price hint for free transactions

[miloszm]

Summary

In order to be able to execute deposit-free free transactions, we need to assume some gas limit and some gas price hint, which are normally, in paid transactions, given by the user. Block generator needs to be able to obtain gas limit and to calculate gas price from a price hint (a ratio to the average gas price).

Possible solution design or implementation

Deploy method needs to accept optional arguments for gas limit and gas price hint and store them in contract's metadata, along with contract's owner.

Additional context

This is part of Economic Protocol scenario 3 implementation, related to issues #350 and #353.