Helper function to derive num_accounts child pairs from mnemonics
Substrate derive function cannot be used because the derivation is different than Ethereum’s
https://substrate.dev/rustdocs/v2.0.0/src/sp_core/ecdsa.rs.html#460-470
Function to generate accounts given a mnemonic and a number of child accounts to be generated
Defaults to a default mnemonic if no mnemonic is supplied