SecurityPkg: add RpmcLib and VariableKeyLib public headers

SecurityPkg: add RpmcLib and VariableKeyLib public headers

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2594

RpmcLib.h and VariableKeyLib.h are header files required to access RPMC
device and Key generator from platform. They will be used to ensure the
integrity and confidentiality of NV variables.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Nishant C Mistry <nishant.c.mistry@intel.com>
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>