Decodes a UTF-8 string with MQTT length prefix.
MQTT strings are prefixed with a 2-byte big-endian length.
The buffer containing the length-prefixed string
The offset to start reading from
DecodeResult with decoded string and bytes read, or error
MQTT 5.0 §1.5.4
Decodes a UTF-8 string with MQTT length prefix.
MQTT strings are prefixed with a 2-byte big-endian length.