Support vars cli command

[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.

TODO

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

[kanej]

This needs a redesign at the product level, as we can now support encryption.

[kanej]

Closing in preference to #5621