gsi_util: adding file-related utilities (file_utils)

The first function is UnopenedTemporaryFile, which is similar to
tempfile.TemporaryFile, except an unopened file path is returned
instead of an opened file-like object (fd).

Bug: 70253882
Test: make gsi_util
Test: ./run_test.py
Change-Id: I11de6976a31d8612cb873212b31c3f13a79b1b3e