Changes

esim.saip.FsProfileElement: Add create_file() method

So far we mainly created File() instances when parsing existing
profiles.  However, sometimes we want to programmatically create Files
and we should offer a convenience helper to do so, rather than asking
API users to worry about low-level details.

Change-Id: I0817819af40f3d0dc0c3d2b91039c5748dd31ee2