UnitTestFrameworkPkg: Add googletest submodule and GoogleTestLib

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4134



Add submodule for googletest and add GoogleTestLib that is
required for GoogleTest based unit tests. Add GoogleTest
documentation to Readme.md along with a port of the sample
unit test to the GoogleTest style.

A few typos in Readme.md are also fixed.

Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Michael Kubacki <mikuback@linux.microsoft.com>
Acked-by: Leif Lindholm <quic_llindhol@quicinc.com>