Bugfix: Length fields missing as prefix to binary fields

[vanrein]

Related to #692 which tries to including binary signatures in the Trailer.

There is support for one length prefix, namely for XML data. The same code can be used for other lenghts, albeit specific to Header, Body, Trailer parts of the message. This was implemented in

https://gitlab.com/arpa2/quickfixgo/-/commit/4606908ab723121b87d3885f0eae15eb2c77d4bc

You are invite to incorporate this into QuickFIX/Go if you think it is helpful. Me being new to the software, I suspect you may have style requirements that I am unaware of, so I will not make a PR unless you think it is helpful.