Support passing in a script/command to get the keys rather than reading from a file

[nogweii]

This is, for example, to be able to retrieve the age keys from a password manager or other secret storage engine. (In my usecase, from KDE's KWallet. But it could also be replaced with MacOS's Keychain, etc.)

Another option, rather than supporting arbitrary code execution, is to support using a platform's native secret storage. Something like go-keychain.