Ext4Pkg: Add uninitialized extents support

Uninitialized extents are special extents that have blocks allocated,
but are specified as uninitialized and therefore, reads behave the
same as file holes (reads 0), while writes already have blocks allocated.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>