[Layer Scanning] Add FileRequirer to the image Config object to allow users to specify files that should be unpacked during Image object creation. Only regular files are handled by the FileRequirer in this cl. Symlinks will be handled in another cl.

[Layer Scanning] Add FileRequirer to the image Config object to allow users to specify files that should be unpacked during Image object creation. Only regular files are handled by the FileRequirer in this cl. Symlinks will be handled in another cl. #1936