We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
If the domain/group/key doesn't exist in the config, exit with
non-zero status and don't print out anything.
Previously the CLI would print a single empty line if the config
value was not found with LibConfig. Now, we use the proper
`Config::Client::the().read_string()` API which can return an
`Optional` type indicating failure.`
