secret: read base64-encoded encryption keys from secret file
Created by: unknwon
Quoting problem statement from #14661:
Editing raw bytes for the secret key file is a strange experience, especially if we also want admin to "join" two arrays of bytes with a ASCII comma.
Instead, base64-encoded string is a good "container" for secrets, and much easier to deal with when we expect admin to do some editing with it.
Fixes #14661