Fortify: Add THREAT_MODEL.md for epplus intake Fixes: b/548455972 This enables the Fortify scanner on our third_party/epplus GoB repo. Change-Id: I75692f479510cefbe90a43cf4032623ecf0efce7 Reviewed-on: https://gnocchi-internal-review.git.corp.google.com/c/third_party/epplus/+/313344 Autosubmit: Hughes Hilton <hugheshilton@google.com> Reviewed-by: Dmitry Riegle <riegle@google.com> Reviewed-by: Trevor Ryland <tryland@google.com>