UnitTestFrameworkPkg: Add subhook submodule required for gmock

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4389

Add subhook submodule that is required to hook internal functions
when using gmock.

    https://github.com/Zeex/subhook

Add SubhookLib library class and SubhookLib library instance.
Include the SUBHOOK_STATIC define in the SubhookLib INF file so
it builds as a static library. Also include the SUBHOOK_STATIC
define in SubhookLib.h so all modules using SubhookLib properly
link SubhookLib as a static library.

Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Signed-off-by: Chris Johnson <chris.n.johnson@intel.com>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Oliver Smith-Denny <osde@linux.microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

UnitTestFrameworkPkg/UnitTestFrameworkPkg.dec

@@ -20,6 +20,7 @@
   Library/CmockaLib/cmocka/include
   Library/GoogleTestLib/googletest/googletest/include
   Library/GoogleTestLib/googletest/googlemock/include
+  Library/SubhookLib/subhook
 
 [Includes.Common.Private]
   PrivateInclude
@@ -34,6 +35,7 @@
   ## @libraryclass GoogleTest infrastructure
   #
   GoogleTestLib|Include/Library/GoogleTestLib.h
+  SubhookLib|Include/Library/SubhookLib.h
 
 [LibraryClasses.Common.Private]
   ## @libraryclass Provides a unit test result report