Change argument of appendBinaryStringInfo from char * to void *

Change argument of appendBinaryStringInfo from char * to void *

There is some code that uses this function to assemble some kind of
packed binary layout, which requires a bunch of casts because of this.
Functions taking binary data plus length should take void * instead,
like memcpy() for example.

Discussion: https://www.postgresql.org/message-id/flat/a0086cfc-ff0f-2827-20fe-52b591d2666c%40enterprisedb.com