Signing a generic byte-array can (and will be) used
by more than the existing PDF signing code, hence
the move into comphelper from vcl and ourside of
the PDF-specific logic.

Change-Id: I7257b5218c6ba37960c6a013746eb387917a23a4
Reviewed-on: https://gerrit.libreoffice.org/39717Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>