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>

UnitTestFrameworkPkg/UnitTestFrameworkPkg.dec

@@ -16,11 +16,15 @@
   PACKAGE_VERSION   = 1.00
 
 [Includes]
+  Include
   Library/CmockaLib/cmocka/include
+  Library/GoogleTestLib/googletest/googletest/include
+  Library/GoogleTestLib/googletest/googlemock/include
 
 [Includes.Common.Private]
   PrivateInclude
   Library/CmockaLib/cmocka/include/cmockery
+  Library/GoogleTestLib/googletest/googletest
 
 [LibraryClasses.Common.Private]
   ## @libraryclass Allows save and restore unit test internal state
@@ -35,6 +39,10 @@
   #
   UnitTestBootLib|PrivateInclude/Library/UnitTestBootLib.h
 
+  ## @libraryclass GoogleTest infrastructure
+  #
+  GoogleTestLib|Include/Library/GoogleTestLib.h
+
 [Guids]
   gUnitTestFrameworkPkgTokenSpaceGuid = { 0x833d3aba, 0x39b4, 0x43a2, { 0xb9, 0x30, 0x7a, 0x34, 0x53, 0x39, 0x31, 0xb3 } }