Add MountedKey encryption.Key implementation
Created by: arussellsaw
This PR adds a new 'mounted' encryption.Key implementation that loads an encryption key either from an environment variable or a file, this is intended for users that either don't have access to or don't want to use cloud encryption APIs
this was co-authored with @indradhanush a week or two ago, just getting round to adding tests and PRing now.
closes #19134 (closed)