Support config var usage from Hardhat console

[kanej]

Config vars are by default encrypted if using the keystore. We want to avoid leaking the password into the console history.
For the moment we are throwing an error if a config var is used within the console we throw an error. This limitation should be removed.
This will effect more than console, indeed it will effect everything that uses requestSecretInput from hre.interruptions.
The problem is that our implementation of requestSecretInput doesn't prevent the stdin to flow into the console. We should investigate how to fix that.