Add a way to set the vsync_interval CVar from code (#5813)

* Add a way to set the render vsync from code

Signed-off-by: rgba16f <82187279+rgba16f@users.noreply.github.com>

* Update change with PR feedback. Missing whitespace plus a comment on why the assignment is outside the value changed check

Signed-off-by: rgba16f <82187279+rgba16f@users.noreply.github.com>

Code/Framework/AzFramework/Tests/Mocks/MockWindowRequests.h

@@ -36,6 +36,7 @@ namespace UnitTest
         MOCK_METHOD0(ToggleFullScreenState, void());
         MOCK_CONST_METHOD0(GetDpiScaleFactor, float());
         MOCK_CONST_METHOD0(GetSyncInterval, uint32_t());
+        MOCK_METHOD1(SetSyncInterval, bool(uint32_t));
         MOCK_CONST_METHOD0(GetDisplayRefreshRate, uint32_t());
     };
 } // namespace UnitTest