Add scanner.ScanFile, ScanReader

What does this MR do?

Add new methods scanner.ScanFile and scanner.ScanReader. These methods are needed for gemnasium-maven and gemnasium-python, respectively.

This is a dependency of:

• gemnasium-maven!15 (merged)
• gemnasium-python!29 (merged)

Related issues

gitlab-org/gitlab#14630 (closed)