This commit introduces the Offset property to libbeat/reader.Message, which stores the total number of bytes read and discarded before generating the message. The Offset field allows inputs to accurately determine how much data has been read up to the message, calculated as Message.Bytes + Message.Offset.

With this new Offset field, the filestream input correctly updates its state to account for data read but discarded by the include_message parser.

(cherry picked from commit 535a174)

# Conflicts:
#	filebeat/input/filestream/environment_test.go