edk2: jansson submodule for edk2 JSON library

Add git submodule "jansson" library, which is the open
source project (https://github.com/akheron/jansson) used
to manipulate JSON data structure. jansson library is
wrapped as edk2 JsonLib and the use cases will be the
edk2 Redfish feature drivers and edk2 port of libredfish
(https://github.com/DMTF/libredfish).
jansson open source project is under MIT license.
(refer to ReadMe.rst under edk2).

Signed-off-by: Abner Chang <abner.chang@hpe.com>

Cc: Andrew Fish <afish@apple.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Nickle Wang <nickle.wang@hpe.com>
Cc: Peter O'Hanley <peter.ohanley@hpe.com>
Reviewed-by: Nickle Wang <nickle.wang@hpe.com>
Acked-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

.gitmodules

@@ -17,3 +17,6 @@
 	path = BaseTools/Source/C/BrotliCompress/brotli
 	url = https://github.com/google/brotli
 	ignore = untracked
+[submodule "RedfishPkg/Library/JsonLib/jansson"]
+	path = RedfishPkg/Library/JsonLib/jansson
+	url = https://github.com/akheron/jansson