chore: Refactor code

[lubej]

• add v5 ethers types;
• improve typings;

[CedarMist]

This adds Ethers v5 back in after it's been carefully removed, so the package goes back to relying on both ethers v5 & ethers v6.

[lubej]

Closing in favor of https://github.com/oasisprotocol/sapphire-paratime/pull/261