hardhat-keystore

[kanej]

Implement the new config variables support, within an initially uncrypted keystore file in the user global Hardhat config directory. This will be provided as a new Hardhat plugin hardhat-keystore.

The details design doc can be found here: https://www.notion.so/nomicfoundation/New-plugin-hardhat-keystore-ec80978a6de54098b60350a436b3e666

There will be follow on work to make the keystore fully encrypted.