Ethereum pallet `rootTransact` support

[boundless-forest]

There is a special dispatch designed to make urgent/necessary calls to the smart contract in dvm system.

• target: the target smart contract address(eth address)
• input: the concatenation of function signature and parameters ethabi encoded binary.

We may need a tool or some docs to outline how to generate and check the input data.

[hackfisher]

I think only users with technical background will use this feature, so simple ways would be just giving tools/documents describing about how to use this dispatch call.

Another solution will be how to verify the input, something similar to the source contract verification and evm call human readable support, which will probably rely on the progress of related support on subscan. cc @yakio