MdePkg: add SmmCpuRendezvousLib.h and SmmCpuRendezvousLibNull implement.

MdePkg: add SmmCpuRendezvousLib.h and SmmCpuRendezvousLibNull implement.

REF? https://bugzilla.tianocore.org/show_bug.cgi?id=3912

UefiCpuPkg define a new Protocol with the new services
SmmWaitForAllProcessor(), which can be used by SMI handler
to optionally wait for other APs to complete SMM rendezvous in
relaxed AP mode.

VariableSmm and VariableStandaloneMM driver in MdeModulePkg need
to use this services but MdeModulePkg can't depend on UefiCpuPkg.

Thus, the solution is moving SmmCpuRendezvouslib.h from UefiCpuPkg
to MdePkg and creating SmmCpuRendezvousLib NullLib version
implementation in MdePkg as dependency for the pkg that can't
depend on UefiCpuPkg.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Signed-off-by: Zhihao Li <zhihao.li@intel.com>
Acked-by: Liming Gao <gaoliming@byosoft.com.cn>