Connected service from connect.
KV bucket name, e.g. avenabox.
Key to read, e.g. <site>.<box>.<source>.config.
The value decoded as UTF-8 text. When the key has no entry or its value
is empty, resolves to the literal string "Key value does not exist" rather
than rejecting, so callers must check for it before parsing.
Reads one value from a KV bucket as a string.