output-packet: use void * instead of OutputCtx * for initdata

output-packet: use void * instead of OutputCtx * for initdata

The use of OutputCtx as the data type for initdata was leaking Eve
submodule logic into the low level packet logger. Instead use void *,
as the packet logging module is not concerned with the type of data
here.

Also document this initdata parameter.

Ticket: #7227