Add config setting for expanding SD-ELEMENTs in SyslogCodec

Activating "expand_structured_data" will now prefix the extracted attributes
from structured syslog messages with the SD-ID of the SD-ELEMENT they've been
extracted from, e. g. '[meta key="val"][mySD@12345 key="val"]' would result in
two attributes "meta_key=val" and "mySD@12345_key=val" in the final GELF message.