fix(cast): --json support for erc20 cmds

[0xferrous]

Motivation

Enable using cast erc20 commands in scripts.

Solution

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes

[0xferrous]

I think a more general solution where all cast commands are refactored to return a value that implements std::fmt::Display and serde::Serialize and using that to print the relevant value would be more robust.