BaseTools: Rename String to StringUtils.

BaseTools: Rename String to StringUtils.

For case-insensitive file systems, edk2 String.py collides with the
Python string.py, which results in build errors. This,for example,
applies to building via the Windows Subsystem for Linux from a
DriveFS file system. This patch renames String to StringUtils to
prevent conflicts for case-insensitive file systems.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>