Apply some fixes to properly estimate ethereum tx fees:

- Set the `extension_weight` on the dispatch_info to properly calculate
the fee with pallet_transaction_payment
- Expose the gas_price through Runtime API, just in case we decide to
tweak the value in future updates, it should be read from the chain
rather than be a shared constant exposed by the crate
- add a `evm_gas_to_fee` utility function to properly convert gas to
substrate fee
- Fix some minor gas encoding for edge cases

---------

Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>