Estimate fee

[FabijanC]

Usage changes

• Introduce fee estimation endpoint.
• Check if max_fee too low in invokes and fail if so.

Dev changes

• Test account+fee behavior through a test in the example repository.
• Use cp instead of mv for handling custom hardhat config files.
• Remove set_origin, make origin public

TODO

• Find out if calls also require fee check (and not just invokes)
  • Answer: they don't.