Encrypted Config Variables

[kanej]

Configuration variables requires command line level support to enable recording vars to user local config files.

This requires porting the existing vars commands to v-next:

Out of scope - supporting encryption of files or individual vars. This will be done as a follow up task after a round of product design.

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

TODO

• Vars vnext design #5553
• hardhat-keystore #5579